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/packs/error.js

@@ -0,0 +1,13 @@
+import ready from '../mastodon/ready';
+
+ready(() => {
+  const image = document.querySelector('img');
+
+  image.addEventListener('mouseenter', () => {
+    image.src = '/oops.gif';
+  });
+
+  image.addEventListener('mouseleave', () => {
+    image.src = '/oops.png';
+  });
+});