record username on merging

[jpahullo]

As an admin I want to track also the usernames of the user accounts being merged in order to track it and help making further reports.

Notes

• We have an automated process for merging users.
• I detected the requirement of some kind of reports involving failed mergings and the reason of failing.
• If somehow, to this automated process, we get a username that does not exist yet on Moodle, we do not have the possibility to track it. What we actually do is just put id 0.

Acceptance conditions

• [ ] I track username for both source and target users, together with their ids.