Feature: Shibboleth/SAML SSO

[LTI Provider] Added an authentication backend to log in LTI users

Clean up capa polling to have exponential backoff

Making background pictures printable

Fix dark lang and django.middleware.locale behaviors 

Adds a temporary user_preference key, DARK_LANGUAGE_KEY, to hold the user's dark lang preference.
This preference key is deleted when ?clear-lang is placed in query params.

This change adds a Django authentication backend which, when installed
will authenticate users based on their LTI identity rather than using
a username/password combination. The authentication method determines
first whether a user with the given username exists and, if so, whether
that user is associated with an LTI identity. It also verifies that
the LTI consumer and user ID passed to the LTI launch match those
stored in the LtiUser table. This will always be the case if the
authentication backend is reached through the LTI code, but it
provides an extra guarantee if the backend is called from elsewhere.

Tests would have caught issues raised in LOC-87

LOC-87

Only return languages we've actually released LOC-85
Perform fuzzy matching to greedily serve the best released language LOC-72

Add to dark_lang middleware

Fix hint-problem tab bug TNL-2542

Revert the removal of hint templates.
Move the 'tab' attribute in the new-problem yaml
files out one level, out of the 'metadata' section.

Improve structure cache metrics

Pass xblock parents

Improve the structure cache metrics with a course context, more standardized from_cache tag, and compression sizes

Fix XBlock inheritance ordering. XBlockMixin and derivatives should always come after XBlock and derivatives

User info cookie

Credit provider integration fixups

PHX-40 added the sorting of the most popular coupon codes used.

Remove restrictions on library creation in Studio (SOL-808)

Fixes DOC-1715, video doc is now in the correct RTD project

Gracefully handle credit provider keys with unicode type