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/UserBio.js

@@ -30,7 +30,7 @@ export default class UserBio extends Component {
     let content;
 
     if (this.editing) {
-      content = <textarea className="FormControl" placeholder={app.trans('core.bio_placeholder')} rows="3" value={user.bio()}/>;
+      content = <textarea className="FormControl" placeholder={app.trans('core.user_bio_placeholder')} rows="3" value={user.bio()}/>;
     } else {
       let subContent;
 
@@ -42,7 +42,7 @@ export default class UserBio extends Component {
         if (bioHtml) {
           subContent = m.trust(bioHtml);
         } else if (this.props.editable) {
-          subContent = <p className="UserBio-placeholder">{app.trans('core.bio_placeholder')}</p>;
+          subContent = <p className="UserBio-placeholder">{app.trans('core.user_bio_placeholder')}</p>;
         }
       }