(BEDS-1116) internal subscription grace period

[remoterami]

A convention involving the unused column users_app_subscriptions.expires_at in combination with the active flag is introduced: when a subscription is active and it expired in the past, it’s currently in a grace period. That’s less effort than adding a new inGracePeriod column or something. Should be merged along with gobitfly/eth2-beaconchain-explorer#2998 for stripe.

This was mostly a research task; see payment provider docs for reference:

• Google: subscription end date returned from API changes dynamically based on state, need internal handling
• Apple: Currently used field is correct, there's another one for grace periods (gracePeriodExpirationDate)
• Stripe: Currently the constant 0 is written as end, need to change. More context

Note that for existing subscriptions this feature can only be ensured to work after the user made an update (e.g. paid an invoice, changed plans etc.).