Revert "If feature flag is enabled, replace the old login/registration pages with the new combined login/registration page."

This reverts commit f40447b3c8f3946c2edbd21c92fe743e9f54a39d.

Conflicts:
	lms/urls.py

Revert "Add support for external auth on the combined login/registration page"

This reverts commit 988753395fa7509472b99b420057e151ce213399.

Conflicts:
	lms/templates/courseware/mktg_course_about.html
	lms/urls.py
Fix help modal css selector
Create helper function to remove duplicate code

This repetitive code is especially pronounced in Stanford's fork, where
we have extended the About page to include several additional fields.

This pays down some technical debt by refactoring into a shared helper.
Implement user service to return currently-logged-in user

returns XBlockuser with 2 data layers
fix unrelated test that was failing