Add unit test for database

[Bryant-Tang]

1. The unit test for User was already merged into main. But might need @Dilamissu to check out if there are some missing test case for field like "is_staff" or "is_superuser".
2. Add unit test for UserTag, Image, Workspace, WorkspaceTag, MissionState, Activity, ActivityNotification, Event, Mission.
3. Add UNIT_TEST_ROOT in settings, in order to store all file data generated during running test.
   !!!Notice!!! :
   (1) All files and folders generating during running test should be store in UNIT_TEST_ROOT, for example, using @override_settings(MEDIA_ROOT=settings.UNIT_TEST_ROOT) before class ImageModelTest(TestCase):.
   (2) All files and folders in UNIT_TEST_ROOT should be deleted after running test, and this action should be done by manually writing codes in tearDownClass function, or other ways that can be automatically execute.