Monthly dues

[ErinCall]

depends on #21

After a user has applied to become a member and been approved, they need to provide payment. The monthly dues are $20 unless they're some other amount? Integrate with Stripe to collect recurring payments. A user must either be a Mentor, an Admin, or be paid up in order to interact with the site.

[pythonchelle]

Mentors will be free, so we should have dues-waiving possibilities built in at least for that use case. It's unclear what dues will be set at after we are out of beta, so if this could stay flexible for now that would be excellent. Currently, dues are $20/month suggested donation, scalable up or down (all the way to $0).