Create new API endpoint for email optin

Creating a new Email Opt-In Endpoint in the profile API for ECOM-525, sub task ECOM-685. This hooks into the new User Org Tag model. 

@dianakhuang @rlucioni
This shouldn't be necessary, since test settings inherit from common.  Maybe this setting needs to be added to `cms/envs/common.py` as well, since the app is in `common/djangoapps`?