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/settings/preferences/show.html.haml

@@ -22,7 +22,7 @@
   %hr#settings_publishing/
 
   .fields-group
-    = f.input :setting_default_privacy, collection: Status.visibilities.keys - ['direct'], wrapper: :with_floating_label, include_blank: false, label_method: lambda { |visibility| safe_join([I18n.t("statuses.visibilities.#{visibility}"), content_tag(:span, I18n.t("statuses.visibilities.#{visibility}_long"), class: 'hint')]) }, required: false, as: :radio_buttons, collection_wrapper_tag: 'ul', item_wrapper_tag: 'li'
+    = f.input :setting_default_privacy, collection: Status.selectable_visibilities, wrapper: :with_floating_label, include_blank: false, label_method: lambda { |visibility| safe_join([I18n.t("statuses.visibilities.#{visibility}"), content_tag(:span, I18n.t("statuses.visibilities.#{visibility}_long"), class: 'hint')]) }, required: false, as: :radio_buttons, collection_wrapper_tag: 'ul', item_wrapper_tag: 'li'
 
     = f.input :setting_default_sensitive, as: :boolean, wrapper: :with_label