Merge commit '96bcee66fba582666259d1d7785c3a7f2a72b8aa' into glitch-soc/merge-upstream

Conflicts:
- `.github/workflows/build-nightly.yml`:
  Upstream changed how the nightly builds are versioned, the conflict
  is because glitch-soc modified adjacent lines relative to the docker
  repositories the images are pushed to.
  Applied upstream's changes.
- `app/views/settings/preferences/notifications/show.html.haml`:
  Upstream moved some settings around. Glitch-soc had more settings.
  Applied upstream's changes, moving glitch-soc's extra settings
  accordingly.

lib/mastodon/version.rb

@@ -17,7 +17,7 @@ module Mastodon
     end
 
     def flags
-      ENV.fetch('MASTODON_VERSION_FLAGS', '-beta2')
+      ENV['MASTODON_VERSION_FLAGS'].presence || '-beta2'
     end
 
     def suffix