Primary key renaming

Improved consistency for existing core translation key names.

See flarum/core#265
- Completely overhauled core en.yml
- Replaced existing key names in all core JS files to match
- Extracted a hardcoded string in IndexPage.js
- Combined two app.trans calls in DiscussionControls.js
- Removed hardcoded spaces from LogInModal.js and SignUpModal.js
- Added two new keys from DiscussionControls.js (soft delete)
- Created two new “reused keys” to YML to accommodate same

js/forum/src/components/DiscussionRenamedNotification.js

@@ -20,6 +20,6 @@ export default class DiscussionRenamedNotification extends Notification {
   }
 
   content() {
-    return app.trans('core.discussion_renamed_notification', {user: this.props.notification.sender()});
+    return app.trans('core.notifications_discussion_renamed_text', {user: this.props.notification.sender()});
   }
 }