Feature: Send course "invites" by email

[twavv]

Currently, the only way to add someone to a course on Pathbird is:

• via course token (at the course register page)
• via purchasing a course (for courses that are configured to be purchasable by students)
• via the GraphQL API

All of these methods require a user to have an account on Pathbird already, which can create additional "roundtrips" for the person wanting to add someone else to a course (they have to wait for the other person to create an account).

We should add a way to invite people by email invite (at a customizable access, e.g., inviting other people as course instructors).