Improve email notifications about license limits

Currently, Crowd sends email notifications when a user is within 10% of the maximum number of users allowed under a particular license.

This can be rather annoying if the number of users is static. Instead, Crowd should send emails as the number of users grows - mails should be sent at (say) every percent change within 10%.

It will be nice if we could adjust the threshold and frequency the email will be sent out as well, as we do not need the email to remind us daily