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/templates/admin/logs.html

@@ -33,6 +33,8 @@
                             <th scope="col">{% trans "Host" %}</th>
                             <th scope="col">{% trans "Instance" %}</th>
                             <th scope="col">{% trans "Message" %}</th>
+                            <th scope="col">{% trans "Result" %}</th>
+                            <th scope="col">{% trans "IP" %}</th>
                             </tr>
                     </thead>
                     <tbody class="searchable">
@@ -44,6 +46,8 @@
                                 <td>{{ log.host }}</td>
                                 <td>{{ log.instance }}</td>
                                 <td>{{ log.message }}</td>
+                                <td>{% if log.success is None %}{% trans "Unknown" %}{% elif log.success %}{% trans "Done" %}{% else %}{% trans "Failed" %}{% endif %}</td>
+                                <td>{{ log.ip|default:"" }}</td>
                             </tr>
                         {% endfor %}
                     </tbody>