Hooking the logistration page into the new HTTP endpoint for email opt in

Updating the Login and Registration page to accept the email_opt_in parameter, which will set an organization-wide setting for receiving emails.

Related to the ECOM-525 story, sub task ECOM-680

@rlucioni @wedaly 
Could you use [Underscore.js](http://underscorejs.org/#isUndefined) here to check if `queryParams.emailOptIn` is undefined? It takes care of some corner cases and gotchas related to determining if a variable is undefined. Should be as straight-forward as `_.isUndefined(queryParams.emailOptIn)`.