Make the regular donations feature work 'better'

[equivalentideas]

Presently there are a lot of unknowns in the regular donation process:

• what happens if someone wants to cancel?
• what happens if a card can't process suddenly?
• do we email people when we draw funds?

As part of the move to Stripe ( #178 ) we should get this working properly and make it a nice experience for these kind people.

[henare]

I'm going to test things with Stripe to see how it all works and fix any things that don't look right along the way.

[henare]

Extracted #182.