Finish Implementing Credential Expiration

[mhrivnak]

CredentialType needs a new attribute named something like "duration". When a credential is created, its expiration date should be set that far into the future.

Furthermore, we need a new restriction: when checking for completion of a CredentialType, all required achievements must have been completed no more than duration ago. So if the duration is 1 year, it will:

1. Check that all required achievements have been completed in the past year
2. Set the date_expires attribute to one year from today.

Lastly, we need a daily task to be run by celery that looks for 'granted' credentials that should be set to expired and does so.