Warn before importing more active users than what the license covers

[michitux]

Steps to reproduce:

Import a Confluence package that contains more active users than covered by the license.

Expected result:

There is at least a warning during the import, maybe with some suggestions what could be done, e.g., filter users by some criterion (group?), importing all (new) users as deactivated, or suggestions how to deactive users in Confluence first. It might make sense to also offer an option to force continuing such that the admin can deactivate users later.

Actual result:

The importer imports as many users as in the import package and instead of the result page displays an error that there is no valid license. While not observed in practice with a user-only import, there could use be issues if, e.g., the task manager pro stops processing tasks after the user limit is exceeded, leading to an incomplete import.