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,
```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;
```