TNL-281: reduce amount of JS in marketing iframe

It probably makes sense to not send lms-main_vendor.js for this view since it's 1MB and dramatically slows down the first page render. 

Acceptance criteria:

  * Allow the bit of LMS that displays the registration button on the marketing site to load without the whole LMS javascript coming along with it.

Tested registration with:
  * marketing site: https://dev.edx.org/course/edx/edx-demox-1-demox-4116?lms=http://tymofij.m.sandbox.edx.org (please enable loading remote iframe content by clicking on the shield icon in the location bar)
  * LMS: http://tymofij.m.sandbox.edx.org

JS size decreased from 960k to 100k
Also adds clickable link on the button for those who like to open links in new tabs.

  * Register-and-enrolд for anonymous users
  * Course about page (with big Enroll button) for registered users