Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

* Add a new cookie for user information
* Make marketing cookie names configurable.
* Handle URL reversal when URLs don't exist (in Studio)
* Move cookie code from student/helpers.py into its own module.

Automatically link edX account with social account in login mobile API path

Fix all the 3rd party auth scenarios.

Update pipeline to preserve parameters.

Updating tests

Set marketing site logged in cookie from third party auth.

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.