Subscriptions shouldn't inherit from ERC20Capped

[adamstallard]

Let's enforce the cap in the SubscriptionsMinter only.

Unless BMAIN DAO gives up all minter roles, they can always bypass the cap by simply creating a new kind of Subscriptions with a different cap.

We don't want to give up all minter roles (because we want to be able to upgrade or replace minters), so having an overall cap is meaningless.

The cap is for this sale only.

This also allows us to maintain only one cap variable instead of trying to have two that are in sync.

This also allows us to not care about the overall supply going down when a subscription is activated (burned) if we choose to make subscriptions transferable.