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.

spec/controllers/auth/sessions_controller_spec.rb

@@ -55,7 +55,7 @@ RSpec.describe Auth::SessionsController, type: :controller do
       request.env['devise.mapping'] = Devise.mappings[:user]
     end
 
-    context 'using PAM authentication' do
+    context 'using PAM authentication', if: ENV['PAM_ENABLED'] == 'true' do
       context 'using a valid password' do
         before do
           post :create, params: { user: { email: "pam_user1", password: '123456' } }