Add webmanifest file, icons. Add manifest to index.html

ui/assets/manifest.webmanifest

@@ -0,0 +1,49 @@
+{
+  "name": "Lemmy",
+  "description": "A link aggregator for the fediverse",
+  "start_url": "/",
+  "display": "minimal-ui",
+  "background_color": "#222222",
+  "icons": [
+    {
+      "src": "/static/assets/icons/icon-72x72.png",
+      "sizes": "72x72",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/assets/icons/icon-96x96.png",
+      "sizes": "96x96",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/assets/icons/icon-128x128.png",
+      "sizes": "128x128",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/assets/icons/icon-144x144.png",
+      "sizes": "144x144",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/assets/icons/icon-152x152.png",
+      "sizes": "152x152",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/assets/icons/icon-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/assets/icons/icon-384x384.png",
+      "sizes": "384x384",
+      "type": "image/png"
+    },
+    {
+      "src": "/static/assets/icons/icon-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ]
+}

ui/src/index.html

@@ -6,6 +6,9 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
 
+    <!-- Web app manifest -->
+    <link rel="manifest" href="/static/assets/manifest.webmanifest">
+
     <!-- Icons -->
     <link rel="shortcut icon" type="image/svg+xml" href="/static/assets/favicon.svg" />
     <link rel="apple-touch-icon" href="/static/assets/apple-touch-icon.png" />