Project import can exceed number of users only if the limit was not reached beforehand

Scenario tested:

• User limit not reached prior to import: DefaultProjectImporterManager spawns one thread per new user, all (or most) threads conclude that are seats available and add the new users to jira-use group, exceeding the license on the way.
• User limit reached prior to import: If the user limit was reached before the import all threads conclude there are no seats available and no user is added to the jira-use group.

This behaviour is not consistent, project import should be able to exceed the license or not regardless of previous state.

It happens both with roles enabled and disabled