Improve License Editing to add or delete seats

If the number of seats change for license this goes through and only deletes seats which are not assigned to a user.
If there are no available seats to delete it returns an error message (which looks like was already in the lang file)

Adding seats is easy and it just does it. 

I've logged the deletion and addition of seats along the way.


** This is my first real pull request, I appreciate any help in making it better.