feat(logs): admin changes and logins are recorded on the Logs page

The Logs page shows who added, changed or deleted a compute, from web
or API, and every login, failed login, lockout and logout, with the
client address and a Result column (Done, Failed, Unknown for older
rows). A failed change is recorded with its reason and the error still
reaches the user; a refused one is recorded too.

- logs.audit.write saves each row in its own savepoint and never
  raises, so a log write cannot break or undo the action it records.
  The audit() context manager writes one row when the change ends.
- Logs gains success and ip; user, host and instance are wider and
  date is indexed. The page orders by date then id.
- A compute edit names the changed fields; an edit that changes
  nothing writes no row, and an empty password field is no change.
- A compute's VMs leaving the panel get a system row each: owners
  kept on the Removed VMs page, moved to another compute, restored,
  left without owners, or the removed record expired.
- Job rows carry the result. A job's log entry that cannot be
  computed is logged and skipped instead of rolling back the finish
  (test_service.py changes this on purpose).
- Logins are recorded by signal receivers; CustomLoginView is removed
  and the plain LoginView is used. A wrong OTP code is a failed login,
  counted by django-axes toward the lockout.

Panel records and host-side objects follow in the next layers.

instances/tests/test_missing_fields.py

@@ -17,6 +17,7 @@ from logs.models import Logs
 class MissingFieldTestCase(TestCase):
     def setUp(self):
         self.client.force_login(get_user_model().objects.create_superuser("mf", "mf@example.com", "pw"))
+        Logs.objects.all().delete()  # the login's row
         self.compute = Compute.objects.create(name="mf-c", hostname="127.0.0.1", login="", password="", type=4)
         self.instance = Instance.objects.create(
             compute=self.compute, name="mf-vm", uuid="11111111-2222-3333-4444-555555555555"