This hardcoded the db_alias fetched from schema_editor and forces django
to try and migrate any second database you use, rather than routing to
the default database.  In testing a build from scratch, these do not
appear needed.

Using using() prevents us from using multiple databases behind edxapp.

Additionally - add back a removed backwards migration from certificates
0003.  I have no idea why this was dropped in the 1.8 upgrade.

We need to be sure the migrations will work even in the presence of data
from the future.  get_or_create is a problem, because if the data
already exists, there could be more than one record, even if this
migration only creates one.

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

Check if the request's user is authenticated before attempting to
get their user preferences. Otherwise, log warnings appear indicating
the UserNotFound error was caught.

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.

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

This reverts commit 09425c3d, reversing
changes made to 410b9282.

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

Also, I replaced a number of tearDown methods with addCleanup instead.
And also remove some unneeded patch.stopall() calls.

LMS-2644

The old-style chinese language codes used in django 1.4 series are now deprecated, as discussed here:  https://code.djangoproject.com/ticket/18419.
Although majority of browsers still use the old-style language codes, some new browsers such as IE11 in Windows 8.1 start to use the new-style names instead of old-style ones which makes the current chinese translations of edX can't work properly under these browsers.
As there is no easy way for edX to change a higher version of django, I think we could do some mapping here so that the users who use IE11 in Windows 8.1 or other browsers that use the new-style names can view correct translations.
Also, this fix will make users who use one of the language codes:  zh-hk ,  zh-sg  and  zh-mo  can see the correct translations.

To mark a language as dark-launched, add it to the DARK_LANGUAGES
django conf setting. To activate a dark-launched language, set he
query parameter `preview-lang` to the language code on any url.

[LMS-2045]
[LMS-2077]
[LMS-2076]