Create registration with OpenEDX api integration

The sign up feature should be altered to do registration with edx.

My suggestion is that we require users to sign up with the same email as on edx, and then do an edx api check on the email address and if user is found with the email and they are enrolled in the course, the account creation is allowed as normal. We should probably store the edx user id with the account.

See https://courses.edx.org/api-docs/#/ for edx api. Api key could be stored in rails credentials file. EDX endpoint could be user/v1/accounts with email flag (though it says "staff only", talk to Udo)

Security concerns:

• (can we) Prevent leaking edx "there is an account on this email address information"