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/DiscussionRenamedPost.js

@@ -14,7 +14,7 @@ export default class DiscussionRenamedPost extends EventPost {
   }
 
   descriptionKey() {
-    return 'core.discussion_renamed_post';
+    return 'core.post_stream_discussion_renamed_text';
   }
 
   descriptionData() {