Add scholarship verification method of unlocking content

[brettdh]

Motivation

Same as theliturgists/app#235, but with the actual access control.

Plan of action

• Enable users to enter their email address
• Confirm that they own the email address
• If the email address is in the list of scholarship recipients, unlock all content
  • Verify membership in MailChimp list, via its API

Scope

This work includes generalizing the access control in a similar way to how both the app and the backend verify a Patreon pledge. This work should probably be done in the patreon-pledge package and just use it in the app and backend.