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.

logs/models.py

@@ -1,12 +1,14 @@
-from django.db.models import CharField, DateTimeField, Model
+from django.db.models import BooleanField, CharField, DateTimeField, GenericIPAddressField, Model, TextField
 from django.utils.translation import gettext_lazy as _
 
 class Logs(Model):
-    user = CharField(_("user"), max_length=50)
-    host = CharField(_("host"), max_length=50, default="-")
-    instance = CharField(_("instance"), max_length=50)
-    message = CharField(_("message"), max_length=255)
-    date = DateTimeField(_("date"), auto_now=True)
+    user = CharField(_("user"), max_length=150)
+    host = CharField(_("host"), max_length=255, default="-")
+    instance = CharField(_("instance"), max_length=120)
+    message = TextField(_("message"))
+    date = DateTimeField(_("date"), auto_now_add=True, db_index=True)
+    success = BooleanField(_("success"), null=True)
+    ip = GenericIPAddressField(_("IP address"), null=True)
 
     def __str__(self):
         return self.instance