Change CLIENT_URL to CLIENT_ORIGIN

[berziiii]

via @gaand.

Heroku looks for the host origin which only consists of scheme, host, and port. This means that for a deployed github client, Heroku's CLIENT_URL config var should be https://<%github_username%>.github.io. It will not accepted requests from https://<%github_username%>.github.io/......

The browser will return an error saying the header is wrong and therefore will not accept the request. Key for client/api deployment for future cohorts.