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/lib/helpers/username.js

@@ -6,7 +6,8 @@
  * @return {Object}
  */
 export default function username(user) {
-  const name = (user && user.username()) || app.trans('deleted');
+  // Core Key Reorganization: Added core namespacing to the key.
+  const name = (user && user.username()) || app.trans('core.deleted_username');
 
   return <span className="username">{name}</span>;
 }