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

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.