Initial setup for new enrollment API

Quick tweaks to the API

Aggregating course modes data.

Fixing the POST request endpoints.
Merge pull request #5606 from edx/sanchez/create-enrollment-api

WIP Enrollment API -- Merge to Reg-Login Form branch only
Merge pull request #5656 from edx/will/logistration-js-work

Combined login/registration JS work
ECOM-408 Added grid-settings to override the default global box-sizing style
ECOM-408 Added breakpoint variables to _grid-settings.scss
Merge pull request #5659 from edx/alasdair/combine-reg-login-form

ECOM-408 Added grid-settings to override the default global box-sizing s...
ECOM-531 Styling the forms
ECOM-531 Styling the buttons
ECOM-531 updating grid column counts in media queries and added some more classes to style against instead of elements
ECOM-531 tweaking margins
Merge pull request #5681 from edx/alasdair/logistration-styling

Alasdair/logistration styling - merge approved by Will
Integrate third party auth into the combined login/registration page.

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.