Fix all the 3rd party auth scenarios.

Update pipeline to preserve parameters.

Updating tests

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

This reverts commit f40447b3.

Conflicts:
	lms/urls.py

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

This reverts commit 98875339.

Conflicts:
	lms/templates/courseware/mktg_course_about.html
	lms/urls.py

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

Also fixes failing Python unit tests

Set marketing site logged in cookie from third party auth.

Change third party auth login failure code to a 401, to detect authentication
success with no linked account.

If already authenticated, redirect immediately to the dashboard.

Use "Location" header correctly for 302 redirects from student views.

Add utility functions for simulating a running third-party auth pipeline.

Add a utility function for checking whether third party auth is enabled.

Respect default values sent by the server

Add Python APIs for account/profile information to user_api
Updating profile page to have social linking

Authors: Renzo Lucioni, Alasdair Swan, Stephen Sanchez, Will Daly

third_party_auth contains a working settings mechanism, the start of the provider interface + 3 implementations (Google, Mozilla Persona, LinkedIn), and a stub for the auth pipeline. Modified existing lms settings files to use but deactivate the module.