Massively simplify API binding logic

Changes the API internals to use normal Laravel binding which automatically supports nested-models and can determine their relationships. This removes a lot of confusingly complex internal logic and replaces it with standard Laravel code.

This also removes a deprecated "getModel" method and fully replaces it with a "parameter" method that does stricter type-checking.

tests/Integration/Api/Application/Users/UserControllerTest.php

@@ -201,18 +201,6 @@ class UserControllerTest extends ApplicationApiIntegrationTestCase
         $this->assertAccessDeniedJson($response);
     }
 
-    /**
-     * Test that a users's existence is not exposed unless an API key has permission
-     * to access the resource.
-     */
-    public function testResourceIsNotExposedWithoutPermissions()
-    {
-        $this->createNewDefaultApiKey($this->getApiUser(), ['r_users' => 0]);
-
-        $response = $this->getJson('/api/application/users/nil');
-        $this->assertAccessDeniedJson($response);
-    }
-
     /**
      * Test that a user can be created.
      */