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/UsersSearchSource.js

@@ -22,7 +22,7 @@ export default class UsersSearchResults {
     if (!results.length) return '';
 
     return [
-      <li className="Dropdown-header">{app.trans('core.users')}</li>,
+      <li className="Dropdown-header">{app.trans('core.search_users_heading')}</li>,
       results.map(user => (
         <li className="UserSearchResult" data-index={'users' + user.id()}>
           <a href={app.route.user(user)} config={m.route}>