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
Good question. I'd say it should be the responsibility of the caller above this function, otherwise we get into the java-developer instinct of calling isinstance() on every argument of the function