Payment: Use Stripe Checkout on client

Introduce the notion of receiving
payments from clients.

Start with subscription plans, using
Stripe's payment platform.

This commit also introduces a first
draft at a logo.
Add 'stripe' dependency

... for payments.
Security: Note an issue with saving user docs

People can put whatever they want in their
user documents. This should not be the case.
Payment: Send subscriptions to Stripe

Pass the Stripe tokens we get from
the client along to the Stripe API,
to activate subscriptions and receive
payments.

In other words, receive money.
Profile: Show active subscription
Payment: Subscriptions can be cancelled