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/DiscussionList.js

@@ -48,7 +48,7 @@ export default class DiscussionList extends Component {
       loading = LoadingIndicator.component();
     } else if (this.moreResults) {
       loading = Button.component({
-        children: app.trans('core.load_more'),
+        children: app.trans('core.discussion_list_load_more_button'),
         className: 'Button',
         onclick: this.loadMore.bind(this)
       });