Remove StripeAuth everywhere from the API

[joshsmith]

Problem

We need to remove the StripeAuth model and everything related to it.

We are creating Stripe managed accounts and no longer need OAuth to be able to connect to Stripe.

Subtasks

• Remove the StripeAuth model and tests (do not have a table to drop since it's virtual)
• Remove from web/models/abilities.ex
• Remove controller and tests
• Remove from router
• Remove view and tests
• Remove reference to Stripe Auth from the API blueprint
• Remove CodeCorps.StripeTesting.Connect.OAuth
• Remove references to Connect.OAuth from StripeConnectAccountService
• Remove anywhere that mentions the stripe_redirect_uri (including mention in USAGE.md)