getIsSubscriberByEmail check DB instead of stripe

[howardchung]

Currently this call requires checking against the Stripe API on every request, which won't scale forever.

We currently cache the subscriber list in the database (syncSubs background process), so we can check the DB instead.

This doesn't update immediately though (it runs on an interval), so we have to make sure that new subs get this updated so they see the sub effects right after subscribing