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/javascript/mastodon/features/compose/components/compose_form.js

@@ -31,6 +31,10 @@ const messages = defineMessages({
 export default @injectIntl
 class ComposeForm extends ImmutablePureComponent {
 
+  static contextTypes = {
+    router: PropTypes.object,
+  };
+
   static propTypes = {
     intl: PropTypes.object.isRequired,
     text: PropTypes.string.isRequired,
@@ -85,7 +89,7 @@ class ComposeForm extends ImmutablePureComponent {
       return;
     }
 
-    this.props.onSubmit();
+    this.props.onSubmit(this.context.router.history);
   }
 
   onSuggestionsClearRequested = () => {