Add the option to set a static expiry date for group access

Setting the number of days that a registrant has group access doesn't make sense for groups with static start and stop dates (such as fitness challenges, etc). As it stands now, participants sign up on different days prior to the start of the program and, in turn, each participant's "end date" will be different, based on when they registered.

The option to add a static end date will ensure that all participants' access will end at the same time.