log password change events

see https://trello.com/c/BkAWL3gn/73-empty-passwords-5-0-5

The log of password changes made after this PR should all be easily viewable via `psql`,
```sql
SELECT u.omename, e.time from experimenter u, eventlog l, event e WHERE l.event = e.id AND e.experimenter = u.id AND l.action = 'PASSWORD' ORDER BY e.time;
```

http://ci.openmicroscopy.org/view/5.0/job/OMERO-5.0-merge-integration-python/lastCompletedBuild/testReport/test.integration.test_admin/TestAdmin/ should pass but first check that the 5.0 merge job suite has actually been run (now it is manually triggered).

--no-rebase