The main django template needs header and footer feature flags.

This is a fix for ECOM-216

A number of django templates in the LMS extend the main_django.html template, which needs to adhere to the new feature flags for the edX header and footer. Also adding a setting to the context that checks if the site is edx.org, and chooses to display the open source footer if not.

@sarina @dianakhuang  -- I'd appreciate a review if able (also shortcuts.py seemed like the right place for the context processors but thought maybe you guys may know a more suitable place?)

@chrisndodge -- I noticed there is a microsite CSS override in main_django.html, I'd like to circle back with you to make sure this does not have a negative impact.

Changes can be seen on steve.m.sandbox.edx.org (where in everything should look like the older header and footer, specifically, the edx.org footer)
I wonder if the feature flag `IS_EDX_DOMAIN` could be used here. We set this to True in the stage and prod envs