Paid Content: add multiple subscription levels

### Impacted plugin

Jetpack

### What

When setting up “Paid Content” rather than “Select a subscription”, it should be “Select subscriptions”. And rather than picking one, it should allow (check box style) to select more than one subscription.

### How

Many places, clubs, museums, even WordPress, have several “membership levels”. It could be “regular member”, and “senior member” (offered at a lower price. It could be “bronze”, “silver”, “gold”, memberships that offer additional features. Similarly, websites, including mine, might wish to offer several levels, at different prices, with some material accessible to all levels, other offerings restricted to higher levels.

That makes it desirable that “Paid Content” blocks be accessible to not just one “subscription level”. When setting up “Paid Content” rather than “Select a subscription”, it should be “Select subscriptions”. And rather than picking one, it should allow (check box style) to select more than one subscription.

Not a huge change, but a very significant one.

When offering products or services it is more effective when the buyer is given a choice of “which one” not just “this or none”. Picking options from several levels is almost universal – “Paid Content” would benefit if it did likewise.

The issue concerns Jetpack’s Paid Content setup, specifically the subscription selector and the resulting access check. Start by locating the Paid Content block and its subscription-selection entry point; done means the UI supports selecting multiple subscription levels and content is accessible to any selected level.

Written by the indexing model from the issue text.