Add checkbox for admins to prevent users from being added above their purchased user count

Problem to solve

Customers do not have enough autonomy to say if they don't want to go over their purchased seat count, some customers have provided feedback that they don't want surprises at their bill and want to have full control over how and when users get added.

Proposal

Add checkbox for admins to prevent users from being added above their purchased user count at the instance level. Even admins cannot accidentally add a user unless they uncheck this box first. This checkbox essentially acts as a guard rail against any accidental billing hiccups as a result via bulk user import/LDAP sync or anything like that.