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/PostsUserPage.js

@@ -51,7 +51,7 @@ export default class PostsUserPage extends UserPage {
       footer = (
         <div className="PostsUserPage-loadMore">
           {Button.component({
-            children: app.trans('core.load_more'),
+            children: app.trans('core.user_posts_load_more_button'),
             className: 'Button',
             onclick: this.loadMore.bind(this)
           })}