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.

app/Http/Requests/Api/Application/Users/UpdateUserRequest.php

@@ -11,7 +11,7 @@ class UpdateUserRequest extends StoreUserRequest
      */
     public function rules(array $rules = null): array
     {
-        $userId = $this->getModel(User::class)->id;
+        $userId = $this->parameter('user', User::class)->id;
 
         return parent::rules(User::getRulesForUpdate($userId));
     }