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.

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

MA-879 Fix bug in course_start_datetime_text

Release merge 06-24-2015

mattdrayer/SOL-449: Add flaky decorator to bok choy test