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.

config/environments/production.rb

@@ -13,7 +13,7 @@ Rails.application.configure do
   # Full error reports are disabled and caching is turned on.
   config.consider_all_requests_local       = false
   config.action_controller.perform_caching = true
-  config.action_controller.asset_host      = ENV['CDN_HOST'] if ENV.key?('CDN_HOST')
+  config.action_controller.asset_host      = ENV['CDN_HOST'] if ENV['CDN_HOST'].present?
 
   # Ensures that a master key has been made available in either ENV["RAILS_MASTER_KEY"]
   # or in config/master.key. This key is used to decrypt credentials (and other encrypted files).