Implement bulk enrollment view

[gsidebo]

PR #178 (issue #112) added a form for sending bulk enrollment emails. Users receive a link in these emails that points to /enroll/?code=<coupon_code>&product=<product_id>. Right now there is a dummy view handling that URL. We need to implement the actual view to allow users to fully enroll in a course from that email link.

Acceptance Criteria:

• Implement an enrollment view that takes a coupon code and product id in the querystring
  • If the user is logged in, redirect to the checkout page
  • If the user is not logged in redirects to the /login endpoint with a next parameter
  • A dummy enrollment view was added in Added admin-only bulk enrollment form #178 (ecommerce.views.anon_enrollment_view, url: /enroll/). The actual implementation should replace this view.
• Ensure that a user can complete checkout and enroll in the course/program on edX
• (possibly redundant) Add a nullable user column to ecommerce.models.BulkEnrollmentDelivery (foreign key to User), and set the value when a user successfully completes enrollment

[pdpinch]

I think this should use the same URL end point as described in #121