We have four rules on the participants table right now:

 1. `bitcoin_addresses` logs into `bitcoin_addresses`
 2. `log_api_key_changes` logs into `api_keys`
 3. `log_goal_changes` logs into `goals`
 4. `log_participant_number` logs into `log_participant_number`

This PR replaces (2) and (3) with calls to the new `add_event` function, and it also uses `add_event` to start logging changes to `username` and `claimed_time`, which were not previously logged. (1) and (4) are unaffected by this PR.