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
Makes sense to me. I'll catch the IntegrityError and log it at warn level. That way, if we see a lot of them, we can establish what's going on. 

Also, given this is a row on a table that only one person should be accessing, I'm hoping we don't have as many scenarios where two people update the same row.