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

Conflicts:
- `config/webpack/configuration.js`:
  Upstream updated the `js-yaml` dependency, which changed how to call it.
  Those changes conflicted because that code is pretty different in glitch-soc
  which has to deal with its more complex theming system.
  Proceeded to the same compatibility changes in glitch-soc's code.
- `package.json` and `yarn.lock`:
  Not really a conflict, just glitch-soc-specific dependencies textually too
  close to some dependencies updated upstream.

lib/mastodon/version.rb

@@ -17,7 +17,7 @@ module Mastodon
     end
 
     def flags
-      'rc3'
+      ''
     end
 
     def suffix