add is_active to role/workspace_member

tracim/models/context_models.py

         return WorkspaceRoles.get_role_from_level(self.user_role.role).slug
 
     @property
+    def is_active(self) -> bool:
+        return self.user.is_active
+
+    @property
     def user(self) -> UserInContext:
         """
         User who has this role, with context data

tracim/tests/functional/test_workspaces.py

         assert user_role['workspace']['slug'] == 'business'
         assert user_role['user']['public_name'] == 'Global manager'
         assert user_role['user']['user_id'] == 1
+        assert user_role['is_active'] is True
         # TODO - G.M - 24-05-2018 - [Avatar] Replace
         # by correct value when avatar feature will be enabled
         assert user_role['user']['avatar_url'] is None

tracim/views/core_api/schemas.py

     workspace = marshmallow.fields.Nested(
         WorkspaceDigestSchema(exclude=('sidebar_entries',))
     )
+    is_active = marshmallow.fields.Bool()
 
     class Meta:
         description = 'Workspace Member information'