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/ReplyPlaceholder.js

@@ -44,7 +44,7 @@ export default class ReplyPlaceholder extends Component {
       <article className="Post ReplyPlaceholder" onclick={reply} onmousedown={triggerClick}>
         <header className="Post-header">
           {avatar(app.session.user, {className: 'PostUser-avatar'})}{' '}
-          {app.trans('core.write_a_reply')}
+          {app.trans('core.post_stream_reply_placeholder')}
         </header>
       </article>
     );