Add user_access_unlocked audit event

Create a user_access_unlocked audit event when the user's access to the
instance is unlocked. There are several ways a user's access can be
unlocked including:

- Unlocked by an admin via users admin page
- Unlocked after the lock expires. In this case the author of the audit
event is set to `GitLab Admin Bot`

EE: true
Changelog: added