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/apps.py

@@ -1,6 +1,38 @@
 from django.apps import AppConfig
 
 
+def _login_row(request, user, message, success):
+    from logs.audit import request_ip, write
+
+    write(user, "-", "", message, success, request_ip(request) if request is not None else None)
+
+
+def logged_in(sender, request, user, **kwargs):
+    _login_row(request, user.get_username(), "Login", True)
+
+
+def login_failed(sender, credentials, request=None, **kwargs):
+    _login_row(request, credentials.get("username", ""), "Login failed", False)
+
+
+def locked_out(sender, request, username, ip_address, **kwargs):
+    _login_row(request, username or "", "Locked out", False)
+
+
+def logged_out(sender, request, user, **kwargs):
+    if user is not None:
+        _login_row(request, user.get_username(), "Logout", True)
+
+
 class LogsConfig(AppConfig):
     name = "logs"
     default_auto_field = "django.db.models.AutoField"
+
+    def ready(self):
+        from axes.signals import user_locked_out
+        from django.contrib.auth.signals import user_logged_in, user_logged_out, user_login_failed
+
+        user_logged_in.connect(logged_in, dispatch_uid="logs.login")
+        user_login_failed.connect(login_failed, dispatch_uid="logs.login_failed")
+        user_locked_out.connect(locked_out, dispatch_uid="logs.locked_out")
+        user_logged_out.connect(logged_out, dispatch_uid="logs.logout")