Add several custom icons (#394)

## Description

Added custom icons that don't meet the requirements of Simple Icons
(especially because they are full-color).

The Airtable and Discourse icons are an upgrade to the single-color
icons in Simple Icons, the others are not available yet at all.

## Type of Change

<!--- Put an `x` in all the boxes that apply: -->

- [x] 🖼️ New icon
- [ ] ✨ New feature (non-breaking change which adds functionality)
- [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 Code refactor
- [ ] ✅ Build configuration change
- [ ] 📝 Documentation
- [ ] 🗑️ Chore

assets/custom-icons/_data/custom-icons.json

@@ -7,6 +7,9 @@
       "title": "Addy.io",
       "slug": "addy_io"
     },
+    {
+      "title": "Airtable"
+    },
     {
       "title": "Anycoin Direct",
       "slug": "anycoindirect"
@@ -43,6 +46,13 @@
       "slug": "controld",
       "hex": "5FD800"
     },
+    {
+      "title": "Discourse"
+    },
+    {
+      "title": "dus.net",
+      "slug": "dusnet"
+    },
     {
       "title": "ente",
       "hex": "1DB954"
@@ -65,6 +75,9 @@
     {
       "title": "ING"
     },
+    {
+      "title": "INWX"
+    },
     {
       "title": "Instagram"
     },
@@ -170,9 +183,15 @@
       "title": "Revolt",
       "hex": "858585"
     },
+    {
+      "title": "service-bw"
+    },
     {
       "title": "SimpleLogin"
     },
+    {
+      "title": "Sipgate"
+    },
     {
       "title": "Snapchat"
     },

assets/custom-icons/icons/airtable.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Airtable</title><path fill="#ffba05" d="M11.992 1.966c-.434 0-.87.086-1.28.257L1.779 5.917c-.503.208-.49.908.012 1.116l8.982 3.558a3.266 3.266 0 0 0 2.454 0l8.982-3.558c.503-.196.503-.908.012-1.116l-8.957-3.694a3.255 3.255 0 0 0-1.272-.257z"/><path fill="#39caff" d="M23.4 8.056a.589.589 0 0 0-.222.045l-10.012 3.877a.612.612 0 0 0-.38.564v8.896a.6.6 0 0 0 .821.552L23.62 18.1a.583.583 0 0 0 .38-.551V8.653a.6.6 0 0 0-.6-.596z"/><path fill="#dc043b" d="M.676 8.095a.644.644 0 0 0-.48.19C.086 8.396 0 8.53 0 8.69v8.355c0 .442.515.737.908.54l6.27-3.006.307-.147 2.969-1.436c.466-.22.43-.908-.061-1.092L.883 8.138a.57.57 0 0 0-.207-.044z"/><path fill-opacity="0.25" d="M10.451,12.997l-2.972,1.434l-7.287,-6.144c0.046,-0.046 0.098,-0.084 0.152,-0.114c0.15,-0.09 0.363,-0.114 0.545,-0.042l9.512,3.769c0.483,0.191 0.521,0.869 0.05,1.097"/></svg>

assets/custom-icons/icons/discourse.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Discourse</title><path d="M11.992 0C5.482 0 0 5.278 0 11.791V24l11.99-.012c6.51 0 11.79-5.481 11.79-11.991C23.78 5.486 18.495 0 11.992 0"/><path d="M12.108 4.564c-4.009.002-7.306 3.301-7.306 7.31 0 1.215.302 2.411.881 3.479L4.36 19.607l4.749-1.072a7.312 7.312 0 0 0 3.002.645c4.009 0 7.308-3.299 7.308-7.308 0-4.008-3.297-7.306-7.305-7.308z" fill="#fff9ae"/><path d="M17.822 16.395a7.307 7.307 0 0 1-8.713 2.128L4.36 19.61l4.834-.571a7.306 7.306 0 0 0 8.712-11.613 7.306 7.306 0 0 1-.084 8.969" fill="#00aeef"/><path d="M17.413 15.006a7.307 7.307 0 0 1-8.443 3.027L4.36 19.61l4.749-1.075A7.306 7.306 0 0 0 16.56 6.078a7.305 7.305 0 0 1 .853 8.928" fill="#00a94f"/><path d="M6.12 15.515a7.308 7.308 0 0 1 11.79-8.091 7.308 7.308 0 0 0-12.227 7.929L4.36 19.607z" fill="#f15d22"/><path d="M5.683 15.353A7.308 7.308 0 0 1 16.56 6.078 7.308 7.308 0 0 0 5.232 15.24l-.869 4.37z" fill="#d0232b"/></svg>

assets/custom-icons/icons/dusnet.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>dus.net</title><circle cx="12" cy="21.799" r="2.201"/><path d="M5.467 0a2.202 2.202 0 1 0 .001 4.403A2.202 2.202 0 0 0 5.467 0M12 0a2.201 2.201 0 1 0 0 4.402A2.201 2.201 0 0 0 12 0m6.533 4.402a2.201 2.201 0 1 0 0-4.402 2.201 2.201 0 0 0 0 4.402M5.467 6.533a2.201 2.201 0 1 0 0 4.402 2.201 2.201 0 0 0 0-4.402m6.533 0a2.2 2.2 0 1 0-.001 4.4 2.2 2.2 0 0 0 .001-4.4m6.533 0a2.201 2.201 0 1 0-.001 4.403 2.201 2.201 0 0 0 .001-4.403M5.467 13.065a2.202 2.202 0 1 0 .001 4.403 2.202 2.202 0 0 0-.001-4.403m6.533 0a2.201 2.201 0 1 0 0 4.402 2.201 2.201 0 0 0 0-4.402m6.533 0a2.202 2.202 0 1 0 0 4.404 2.202 2.202 0 0 0 0-4.404" fill="#eb311b"/></svg>

assets/custom-icons/icons/service-bw.svg

@@ -0,0 +1 @@
+<svg rols="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>service-bw</title><path fill="#f8dd15" d="M7.926 7.962c-.582.086-.083.499-.083.499l.618-.477s.035-.09-.535-.022 M9.571 10.864c.144-1.248-1.379-2.88-1.379-2.88l.592-.794-1.478.407s-1.581-.691-3.946.793a1.717 1.717 0 0 0 0 .989c.097.322.925-.48 1.107.154-.269.078-.548.12-.829.125a1.239 1.239 0 0 1-.758-.196 2.065 2.065 0 0 1-.48-.96c-.073-.356-.342 1.447 1.21 1.447.293.026.588-.004.87-.09-.051.858-.829.215-.829.215v.988s1.258-.163 1.488-.339c-.064.221-.173.819-.173.819l-2.025-.08S.691 9.478 0 11.856l1.581.099 3.651 2.378-1.677 1.28s-2.56-.858-2.464 1.123l3.251.058 5.095-1.184v-.586s1.449-.099 1.747-.099a15.308 15.308 0 0 0 1.862-.778v-3.232c-1.724.202-3.193.269-3.475-.051m-3.859-2.88-.618.493s-.499-.416.084-.499c.582-.084.534.022.534.022z M11.546 8.883c-.298-.601.492-1.683.492-1.683s-2.169.595-1.776 1.981c.304 1.069 2.01.96 2.778.854v-.96c-.618.103-1.334.135-1.494-.192"/><path d="M13.28 14.024v4.056h-.32V5.92h.32v3.146c.247-.045.493-.099.736-.164 0 0 5.203-1.52 5.622-1.6.42-.08 2.269-.396 2.762 1.485.176 1.415-.48 1.776-.995 2.279.883.08 1.184 2.393 1.184 3.068 1.062-.371 1.283.298 1.382.298.099 0-.089 2.179-.089 2.179l-2.762-.096s-.352-1.017 1.28-1.081c-2.275-.692-3.261-2.068-3.261-2.068s-.889 1.104-1.184 1.6c-.294.496-.393 1.642-.393 1.642H14.01c.06-.55.182-.858.588-1.046l1.476.041.387-2.771-.413-.099s-1.449.677-2.768 1.291m0-3.995v.845c2.485-.308 5.388-.857 5.958-.912a1.866 1.866 0 0 1 1.479.396s.598.496.79-.988c.192-1.485-1.974-1.204-2.368-1.088-.393.115-5.705 1.718-5.705 1.718s-.057.012-.154.029"/></svg>