Update API action architecture

- An API action handles a Flarum\Api\Request, which is a simple object
containing an array of params, the actor, and optionally an HTTP
request object
- Most API actions use SerializeAction as a base, which parses request
input according to the JSON-API spec (creates a JsonApiRequest object),
runs the child class method to get data, then serializes it and assigns
it to a JsonApiResponse (standard HTTP response with a
Tobscure\JsonApi\Document as content)
- The JSON-API request input parsing is subject to restrictions as
defined by public static properties on the action (i.e. extensible)
- Also the actor is given to the serializer instance now, instead of
being a static property

src/Api/Actions/Posts/GetsPosts.php

@@ -1,19 +1,17 @@
 <?php namespace Flarum\Api\Actions\Posts;
 
 use Flarum\Core\Models\User;
-use Flarum\Api\Actions\ApiParams;
+use Flarum\Api\JsonApiRequest;
 
 trait GetsPosts
 {
-	protected function getPosts(ApiParams $params, $where)
+	protected function getPosts(JsonApiRequest $request, array $where)
 	{
-		$sort = $params->sort(['time']);
-        $count = $params->count(20, 50);
-        $user = $this->actor->getUser();
+        $user = $request->actor->getUser();
 
-        if (isset($where['discussion_id']) && ($near = $params->get('near')) > 1) {
+        if (isset($where['discussion_id']) && ($near = $request->get('near')) > 1) {
             $start = $this->posts->getIndexForNumber($where['discussion_id'], $near, $user);
-            $start = max(0, $start - $count / 2);
+            $start = max(0, $request->offset - $request->limit / 2);
         } else {
             $start = 0;
         }
@@ -21,10 +19,9 @@ trait GetsPosts
         return $this->posts->findWhere(
             $where,
             $user,
-            $sort['field'],
-            $sort['order'] ?: 'asc',
-            $count,
-            $start
+            $request->sort,
+            $request->limit,
+            $request->offset
         );
 	}
 }