Implement user "bio" field

Perhaps this should be an extension, but it is pretty essential and I
can’t think of many instances where it wouldn’t be wanted. Would be
very easy to extract later on if need be.

src/Core/CoreServiceProvider.php

@@ -118,6 +118,7 @@ class CoreServiceProvider extends ServiceProvider
         Model::setValidator($this->app['validator']);
 
         User::setHasher($this->app['hash']);
+        User::setFormatter($this->app['flarum.formatter']);
     }
 
     public function registerPermissions()