Extend local caching to more file types (dart-lang#5026)

This expands local caching to also cache `webp` images, `webm` videos
(used in home page), `svg` images, as well as `css` and `js` files. We
append cache busts to our styles on build as well as some scripts as
necessary.

Author: parlough

firebase.json

@@ -5,7 +5,7 @@
     "trailingSlash": false,
     "headers": [
       {
-        "source": "**/*.@(jpg|jpeg|gif|png|md|txt|json)",
+        "source": "**/*.@(jpg|jpeg|gif|png|md|txt|json|webp|webm|svg|css|js)",
         "headers": [
           { "key": "Cache-Control", "value": "max-age=3600" },
           { "key": "Access-Control-Allow-Origin", "value": "*" }