Add Letterboxd icon (#464)

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Add Letterboxd icon

<!--- 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

@@ -168,6 +168,9 @@
       "title": "La Poste",
       "slug": "laposte"
     },
+    {
+      "title": "Letterboxd"
+    },
     {
       "title": "Mastodon",
       "altNames": ["mstdn", "fediscience", "mathstodon", "fosstodon"],

assets/custom-icons/icons/letterboxd.svg

@@ -0,0 +1,7 @@
+<svg width="500" height="500" viewBox="0 0 500 500" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M250 342.617C301.195 342.617 342.697 301.289 342.697 250.308C342.697 199.328 301.195 158 250 158C198.805 158 157.303 199.328 157.303 250.308C157.303 301.289 198.805 342.617 250 342.617Z" fill="#00E054"/>
+<path d="M92.6966 342.617C143.892 342.617 185.393 301.289 185.393 250.308C185.393 199.328 143.892 158 92.6966 158C41.5017 158 0 199.328 0 250.308C0 301.289 41.5017 342.617 92.6966 342.617Z" fill="#FF8000"/>
+<path d="M407.303 342.617C458.498 342.617 500 301.289 500 250.308C500 199.328 458.498 158 407.303 158C356.108 158 314.606 199.328 314.606 250.308C314.606 301.289 356.108 342.617 407.303 342.617Z" fill="#40BCF4"/>
+<path fill-rule="evenodd" clip-rule="evenodd" d="M171.348 299.183C162.448 285.01 157.303 268.257 157.303 250.308C157.303 232.36 162.448 215.607 171.348 201.434C180.249 215.607 185.393 232.36 185.393 250.308C185.393 268.257 180.249 285.01 171.348 299.183Z" fill="#556677"/>
+<path fill-rule="evenodd" clip-rule="evenodd" d="M328.652 201.434C337.552 215.607 342.697 232.36 342.697 250.308C342.697 268.257 337.552 285.01 328.652 299.183C319.751 285.01 314.607 268.257 314.607 250.308C314.607 232.36 319.751 215.607 328.652 201.434Z" fill="#556677"/>
+</svg>