Added a license key type model.

Change-Id: I98c6114a8de2f68d826e0434b3aa834da3fdc674
Rename next! to assign_license_keys!

Change-Id: I3a06a1fe0ac89c7ec0af9a799b87125a7adf44a1
Send multiple keys when needed.

This modifies the mailer to send out multiple keys for a single product
when the have multiple license key types.

Change-Id: I12c7ca6a4b7ee48b1418ee4c2eec3c8cca76fd38