UC Flexible Subscriptions

This module makes UC Subscriptions and corresponding products more flexible.

The current functions allow admins to:
- update the next charge date for all subscriptions of a given role. This can be particularly helpful when subscriptions aren't renewed regularly.
- add files from a product to all users of a given role. This is helpful in situations where a recurring fee is added to a product, files are regularly changed on the product, and files need to be granted to current subscribers instead of all product purchasers.