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

Conflicts:
- .github/ISSUE_TEMPLATE/bug_report.md
  Took our version.
- CONTRIBUTING.md
  Updated the embedded copy of upstream's version.
- README.md
  Took our version.
- app/policies/status_policy.rb
  Not a real conflict, took code from both.
- app/views/layouts/embedded.html.haml
  Added upstream's changes (dns-prefetch) and fixed
  `%body.embed`
- app/views/settings/preferences/show.html.haml
  Reverted some of upstream changes, as we have a
  page dedicated for flavours and skins.
- config/initializers/content_security_policy.rb
  Kept our version of the CSP.
- config/initializers/doorkeeper.rb
  Not a real conflict, took code from both.

app/javascript/mastodon/locales/pl.json

@@ -99,6 +99,7 @@
   "confirmations.reply.message": "W ten sposób utracisz wpis który obecnie tworzysz. Czy na pewno chcesz to zrobić?",
   "confirmations.unfollow.confirm": "Przestań śledzić",
   "confirmations.unfollow.message": "Czy na pewno zamierzasz przestać śledzić {name}?",
+  "conversation.last_message": "Last message:",
   "embed.instructions": "Osadź ten wpis na swojej stronie wklejając poniższy kod.",
   "embed.preview": "Tak będzie to wyglądać:",
   "emoji_button.activity": "Aktywność",