add icon for notesnook (#443)

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## Description

add the svg icon for notesnook

<!--- Describe your changes in detail -->

## 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

@@ -195,6 +195,9 @@
     {
       "title": "Njalla"
     },
+    {
+      "title": "Notesnook"
+    },
     {
       "title": "Notion"
     },

assets/custom-icons/icons/Notesnook.svg

@@ -0,0 +1,20 @@
+<svg xmlns="http://www.w3.org/2000/svg"
+  xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 339 339">
+  <defs />
+  <defs>
+    <linearGradient id="b" x1="193.9" x2="198.7" y1="166.8" y2="223.3" gradientTransform="rotate(5 4448 -4204) scale(2.93671)" gradientUnits="userSpaceOnUse" xlink:href="#a" />
+    <linearGradient id="a">
+      <stop offset="0" />
+      <stop offset="1" stop-color="#fff9f9" stop-opacity="0" />
+    </linearGradient>
+    <linearGradient id="c" x1="167.8" x2="270.6" y1="76.9" y2="64.2" gradientTransform="rotate(5 465 -2050) scale(1.50082)" gradientUnits="userSpaceOnUse" xlink:href="#a" />
+  </defs>
+  <g transform="translate(0 42)">
+    <path fill="url(#b)" d="M160 205l154 42-141 44-155-42z" />
+    <path fill="url(#c)" d="M160-35v240l154 42 1-253z" />
+    <path fill="none" stroke-width="1.2" d="M160 205V-35m0 240L18 249m142-44l154 41" />
+    <path d="M84 109l35 54V98l21-7v91l-27 9-35-54v65l-21 6v-91z" />
+    <rect width="86.1" height="12.6" x="185" y="97" fill="#bebebe" ry="2.3" transform="skewY(15) scale(.9669 1)" />
+    <path fill="#bebebe" d="M181 169l99 26 2 3v8c0 1-1 2-2 1l-99-26-2-3v-7c0-2 1-2 2-2zm0-47l99 27 2 2v8l-2 2-99-27c-1 0-2-1-2-3v-7l2-2z" />
+  </g>
+</svg>