Make the ForeiginKey detection more accurate

To handle this case: the project use tastypie and django.
tastypie has a `ForeignKey` field which has the same name
as django's `ForeignKey`.
The issue is the lint trys resolving the `ForeignKey` for the
tastypie `ForeignKey` which cause import error.

In this commit, add a check to ensure the current class of the
`ForeignKey` is a subclass of `Model` of django.

Tested manually
Test case added: func_noerror_foreign_key_in_non_django_class

Author: Tommy Wu

pylint_django/tests/input/func_noerror_foreign_key_in_non_django_class.py

@@ -0,0 +1,25 @@
+"""
+Checks that Pylint raise error when a 'ForeignKey' appears in a
+non-django class
+
+The real case is described as follow:
+The project use tastypie and django.
+tastypie has a `ForeignKey` field which has the same name
+as django's `ForeignKey`.
+The issue is the lint trys resolving the `ForeignKey` for the
+tastypie `ForeignKey` which cause import error.
+"""
+# pylint: disable=missing-docstring
+
+class ForeignKey: # pylint: disable=too-few-public-methods
+    def test_methond(self):
+        pass
+
+
+class Resource: # pylint: disable=too-few-public-methods
+    def test_methond(self):
+        pass
+
+
+class MyTestResource(Resource): # pylint: disable=too-few-public-methods
+    author = ForeignKey('myapp.api.resource', 'xxx')

pylint_django/transforms/foreignkey.py

@@ -16,6 +16,11 @@ def is_foreignkey_in_class(node):
     if not isinstance(node.parent.parent, ClassDef):
         return False
 
+    # Make sure the outfit class is the subclass of django.db.models.Model
+    is_in_django_model_class = node_is_subclass(node.parent.parent, 'Model')
+    if not is_in_django_model_class:
+        return False
+
     if isinstance(node.func, Attribute):
         attr = node.func.attrname
     elif isinstance(node.func, nodes.Name):