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/serializers/initial_state_serializer.rb

@@ -21,6 +21,7 @@ class InitialStateSerializer < ActiveModel::Serializer
       search_enabled: Chewy.enabled?,
       version: Mastodon::Version.to_s,
       invites_enabled: Setting.min_invite_role == 'user',
+      mascot: instance_presenter.mascot&.file&.url,
     }
 
     if object.current_account
@@ -63,4 +64,10 @@ class InitialStateSerializer < ActiveModel::Serializer
   def media_attachments
     { accept_content_types: MediaAttachment::IMAGE_FILE_EXTENSIONS + MediaAttachment::VIDEO_FILE_EXTENSIONS + MediaAttachment::AUDIO_FILE_EXTENSIONS + MediaAttachment::IMAGE_MIME_TYPES + MediaAttachment::VIDEO_MIME_TYPES + MediaAttachment::AUDIO_MIME_TYPES }
   end
+
+  private
+
+  def instance_presenter
+    @instance_presenter ||= InstancePresenter.new
+  end
 end