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.

.circleci/config.yml

@@ -13,6 +13,9 @@ aliases:
           ALLOW_NOPAM: true
           CONTINUOUS_INTEGRATION: true
           DISABLE_SIMPLECOV: true
+          PAM_ENABLED: true	
+          PAM_DEFAULT_SERVICE: pam_test	
+          PAM_CONTROLLED_SERVICE: pam_test_controlled
     working_directory: ~/projects/mastodon/
 
   - &attach_workspace