Massive refactor

- Use contextual namespaces within Flarum\Core
- Clean up and docblock everything
- Refactor Activity/Notification blueprint stuff
- Refactor Formatter stuff
- Refactor Search stuff
- Upgrade to JSON-API 1.0
- Removed “addedPosts” and “removedPosts” relationships from discussion
API. This was used for adding/removing event posts after renaming a
discussion etc. Instead we should make an additional request to get all
new posts

Todo:
- Fix Extenders and extensions
- Get rid of repository interfaces
- Fix other bugs I’ve inevitably introduced

js/lib/utils/truncate.js

@@ -1,5 +1,6 @@
 export default function truncate(string, length, start) {
   start = start || 0;
+  string = string || '';
 
   return (start > 0 ? '...' : '')+string.substring(start, start + length)+(string.length > start + length ? '...' : '');
 }