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/initial_state.js

@@ -15,5 +15,6 @@ export const searchEnabled = getMeta('search_enabled');
 export const maxChars = (initialState && initialState.max_toot_chars) || 500;
 export const invitesEnabled = getMeta('invites_enabled');
 export const version = getMeta('version');
+export const mascot = getMeta('mascot');
 
 export default initialState;