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.

admin/views.py

@@ -207,10 +207,8 @@ def user_unblock(request, pk):
 
 @superuser_only
 def logs(request):
-    l = Logs.objects.order_by("-date")
-    paginator = Paginator(l, int(app_settings.LOGS_PER_PAGE))
-    page = request.GET.get("page", 1)
-    logs = paginator.page(page)
+    paginator = Paginator(Logs.objects.order_by("-date", "-id"), int(app_settings.LOGS_PER_PAGE))
+    logs = paginator.get_page(request.GET.get("page"))
     return render(request, "admin/logs.html", {"logs": logs})