Unit test user app

* adds unit test for User model and forms
* refactors functions from user_checks.py into User class and drops user_checks.py

api/tests/v1/share/test_api_sharing.py

@@ -4,7 +4,6 @@ from django.urls import reverse
 
 from konova.settings import DEFAULT_GROUP
 from konova.tests.test_views import BaseTestCase
-from konova.utils.user_checks import is_default_group_only
 
 
 class BaseAPIV1TestCase(BaseTestCase):
@@ -138,7 +137,7 @@ class APIV1SharingTestCase(BaseAPIV1TestCase):
         # Give the user only default group rights
         default_group = self.groups.get(name=DEFAULT_GROUP)
         self.superuser.groups.set([default_group])
-        self.assertTrue(is_default_group_only(self.superuser))
+        self.assertTrue(self.superuser.is_default_group_only())
 
         # Add only him as shared_users an object
         self.intervention.users.set([self.superuser])