Improve handling of deleted users in Account Manager

Referring to Account Manager -> workspace users -> deleted users.

Two requests: add a (manual) mass operation for anonymising deleted users, and show more information for deleted users, particularly the time the user was deleted.

It would be especially useful to know when the user was deleted. In addition, it would be nice to know from which workspace(s) they were deleted.

This information would be useful for user data handling, particularly anonymising users who were deleted a long time ago. The need is partially covered by the possibility to automatically anonymise users, but the feature needs to be activated separately in each workspace, meaning that it can be a tedious process (not to mention that an admin can disable it).