Add a field to teacher profiles in Mongo for actual permissions

[kwokwilliam]

Make it so anyone can make an account through the /v1/teacher POST endpoint.

This admin: true field in the teacher object in Mongo should be all we need to make sure that the user is truly an admin and can remove users and read the queue.

The very first user will need this manually added in Mongo, but the rest should be available to be added through some sort of patch by admins only, perhaps through /v1/teacher/{teacher_id}.