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/routes.rb

@@ -267,7 +267,12 @@ Rails.application.routes.draw do
       resources :streaming, only: [:index]
       resources :custom_emojis, only: [:index]
       resources :suggestions, only: [:index, :destroy]
-      resources :conversations, only: [:index]
+
+      resources :conversations, only: [:index, :destroy] do
+        member do
+          post :read
+        end
+      end
 
       get '/search', to: 'search#index', as: :search