Store username in the logs

[fsuter]

The logs currently store the id of the user that ran the synchronisations. This requires quite some handling when displaying the logs in the BE module, as the username must be retrieved then. This also complicates sorting and filtering, now that Extbase is not used anymore.

It might also be that a user does not exist anymore when logs are displayed.

As such, it is easier and more solid to store the username and id as a string when writing the log entries. Change the current structure and create a wizard to migrate existing entries.