Move Group to its own namespace

We’ll need to add commands etc. for group management in the future

src/Api/Actions/Groups/IndexAction.php

@@ -1,6 +1,6 @@
 <?php namespace Flarum\Api\Actions\Groups;
 
-use Flarum\Core\Users\Group;
+use Flarum\Core\Groups\Group;
 use Flarum\Api\Actions\SerializeCollectionAction;
 use Flarum\Api\JsonApiRequest;
 use Tobscure\JsonApi\Document;

src/Core/Groups/Group.php

@@ -1,4 +1,4 @@
-<?php namespace Flarum\Core\Users;
+<?php namespace Flarum\Core\Groups;
 
 use Flarum\Core\Model;
 

src/Core/Users/Guest.php

@@ -1,5 +1,7 @@
 <?php namespace Flarum\Core\Users;
 
+use Flarum\Core\Groups\Group;
+
 class Guest extends User
 {
     /**

src/Core/Users/User.php

@@ -1,5 +1,6 @@
 <?php namespace Flarum\Core\Users;
 
+use Flarum\Core\Groups\Group;
 use Flarum\Core\Model;
 use Flarum\Core\Notifications\Notification;
 use Illuminate\Contracts\Hashing\Hasher;
@@ -487,7 +488,7 @@ class User extends Model
      */
     public function groups()
     {
-        return $this->belongsToMany('Flarum\Core\Users\Group', 'users_groups');
+        return $this->belongsToMany('Flarum\Core\Groups\Group', 'users_groups');
     }
 
     /**