Merge branch 'master' into glitch-soc/merge-upstream

Conflicts:
- app/serializers/rest/instance_serializer.rb
- app/views/layouts/error.html.haml
- public/oops.png
  Took our version (same as upstream but without the things that only
  make sense in an animation).

Additional changes:
- app/javascript/flavours/vanilla/theme.yml
  Include upstream's javascript in error pages.

app/javascript/styles/mastodon/basics.scss

@@ -100,12 +100,14 @@ body {
       vertical-align: middle;
       margin: 20px;
 
-      img {
-        display: block;
-        max-width: 470px;
-        width: 100%;
-        height: auto;
-        margin-top: -120px;
+      &__illustration {
+        img {
+          display: block;
+          max-width: 470px;
+          width: 100%;
+          height: auto;
+          margin-top: -120px;
+        }
       }
 
       h1 {