Use current active user count to determine whether license is valid.

Previously, the EE license check used the historical maximum to determine
whether an admin has enough seats for the license. Let's say a customer has a
5-seat license, and sometime during the year he had 20 users. If he/she trims back
to 4 users, the 5-seat license won't work anymore. Right now the workaround is
to issue a license for 20 users.

Closes #550