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/views/admin/settings/edit.html.haml

@@ -26,6 +26,8 @@
       = f.input :thumbnail, as: :file, wrapper: :with_block_label, label: t('admin.settings.thumbnail.title'), hint: t('admin.settings.thumbnail.desc_html')
     .fields-row__column.fields-row__column-6.fields-group
       = f.input :hero, as: :file, wrapper: :with_block_label, label: t('admin.settings.hero.title'), hint: t('admin.settings.hero.desc_html')
+
+  .fields-row
     .fields-row__column.fields-row__column-6.fields-group
       = f.input :mascot, as: :file, wrapper: :with_block_label, label: t('admin.settings.mascot.title'), hint: t('admin.settings.mascot.desc_html')