EasyInvoice - Subscription Plan _Groups_ (Mutually Exclusive Subscriptions)

[MantisClone]

Problem

If a merchant has a few different subscription tiers (ex. Standard, Premium, etc.) for the same service then it should not be possible for a user to subscribe to both Standard and Premium at the same time.

Proposed Solution

Allow the Merchant to make Subscription Plan Groups, in which a subscriber can only be subscribed to 1 of the subscriptions at a time. In other words, mutually exclusive subscriptions.

[MantisClone]

Reconsider priority if Subscriptions gain traction.