Use privacy.backend.AdminPermission.is_admin for testing project admins

Based on feedback:
#1503 (comment)

Author: gregmuellegger

readthedocs/projects/forms.py

@@ -14,6 +14,7 @@
 from readthedocs.redirects.models import Redirect
 from readthedocs.projects import constants
 from readthedocs.projects.models import Project, EmailHook, WebHook
+from readthedocs.privacy.loader import AdminPermission
 
 
 class ProjectForm(forms.ModelForm):
@@ -320,7 +321,7 @@ def clean_subproject(self):
             raise forms.ValidationError((_("Project %(name)s does not exist")
                                          % {'name': subproject_name}))
         subproject = subproject_qs.first()
-        if not subproject.user_is_admin(self.user):
+        if not AdminPermission.is_admin(self.user, subproject):
             raise forms.ValidationError(_(
                 'You need to be admin of {name} in order to add it as '
                 'a subproject.'.format(name=subproject_name)))

readthedocs/projects/models.py

@@ -253,9 +253,6 @@ def sync_supported_versions(self):
                 verbose_name__in=supported).update(supported=False)
             self.versions.filter(verbose_name=LATEST_VERBOSE_NAME).update(supported=True)
 
-    def user_is_admin(self, user):
-        return user in self.users.all()
-
     def save(self, *args, **kwargs):
         first_save = self.pk is None
         if not self.slug:

readthedocs/rtd_tests/tests/test_views.py

@@ -8,6 +8,7 @@
 from readthedocs.builds.constants import LATEST
 from readthedocs.projects.models import Project
 from readthedocs.projects.forms import UpdateProjectForm
+from readthedocs.privacy.loader import AdminPermission
 
 
 class Testmaker(TestCase):
@@ -200,7 +201,7 @@ def test_admins_can_delete_subprojects(self):
 
     def test_project_admins_can_delete_subprojects_that_they_are_not_admin_of(self):
         self.project.users.add(self.user)
-        self.assertFalse(self.subproject.user_is_admin(self.user))
+        self.assertFalse(AdminPermission.is_admin(self.user, self.subproject))
 
         response = self.client.get('/dashboard/my-mainproject/subprojects/delete/my-subproject/')
         self.assertEqual(response.status_code, 302)