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/defaultMessages.json

@@ -1047,6 +1047,15 @@
     ],
     "path": "app/javascript/mastodon/features/compose/index.json"
   },
+  {
+    "descriptors": [
+      {
+        "defaultMessage": "Last message:",
+        "id": "conversation.last_message"
+      }
+    ],
+    "path": "app/javascript/mastodon/features/direct_timeline/components/conversation.json"
+  },
   {
     "descriptors": [
       {