notifications: Use a dynamic SVG favicon for unread counts.

Closes #2304.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

package.json

@@ -32,6 +32,7 @@
     "file-loader": "^6.0.0",
     "flatpickr": "^4.5.7",
     "font-awesome": "^4.7.0",
+    "font-subset-loader2": "^1.1.7",
     "ga-gtag": "^1.0.1",
     "handlebars": "^4.7.2",
     "handlebars-loader": "^1.7.1",
@@ -65,6 +66,7 @@
     "style-loader": "^1.0.0",
     "terser-webpack-plugin": "^4.1.0",
     "turndown": "^6.0.0",
+    "url-loader": "^4.1.1",
     "webfonts-loader": "^7.0.1",
     "webpack": "^4.33.0",
     "webpack-cli": "^3.3.2",