Cache course location translations
Create local memory caching entry for location mapping.
Add set to mocked cache
Only create loc_cache CACHE entry if it doesn't exist in ENV_TOKENS config.
Merge pull request #2337 from edx/dhm/bug_perf_loc

Dhm/bug perf loc
Disable transcript test
Merge pull request #2340 from edx/will/disable-transcript-test

Disable transcript test
Set the html language to the browser's specified language
Merge pull request #2332 from edx/dstgermain/a11y-html-lang

Set the HTML language to the browser's specified language
Pin subunit and testtools libraries to prevent test failures after release of these libraries on 1/29/14
Merge pull request #2351 from edx/will/pin-test-lib-versions

Pin test lib versions (fix to test failures in master)
Add the ability to dark-launch languages

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]
Add config_model, a library for database backed configuration

ConfigurationModels can be managed using the admin site. They are
append-only, and track the user who is making the change, and the time
that the change was made. The configuration is stored in the database,
and cached for performance.

[LMS-1220]
Switch dark_lang to database backed configuration