Studio: revises CSS reference/pipeline architecture for better organization, reduced redundancy, and to handle IE9 selector limits

Was re-introduced in a bad merge.

add middleware to detect students with disabled accounts

add tests

@jzoldak showed me that setting `USE_I18N = DEBUG` didn't actually accomplish
what I had hoped it would -- changing DEBUG didn't also change USE_I18N.
This pull request accomplishes what I was trying to accomplish, without being
quite so clever about it.

We want USE_I18N to be on in development, but off in production. Tying
this setting to the DEBUG setting accomplishes that neatly.

   And include Don's fix for partial course_id lookup.

cleanup

Updates.

Testing changes.

Testing changes.

asset-index progress state cleanjup.

cleanup

* Add unit test coverage for this "exclusion from tracking" logic
* Exclude based on regex match instead of raw string comparison
* Exclude heartbeat requests from tracking

Also added datadog application to commom django apps and set it to use
a local statsd server by default.

due to the MixedModuleStore pending release, we need to make enrollment start/end dates editable in Studio

STUD-669.

Previously the file upload dialog was PDF- and textbook-specific. The
changes are adding parameters to the FileUpload model for the file
type, and adding an onSuccess callback to the UploadDialog view. Also
moved upload-specific SASS into its own file.

The django_nose it is very useful, even outside the test
environment. For example, it lets you to easily run test from
manage.py without additional changes to the test packages.

Studio: adds in new Sass file for Studio-specific
xmodule presentation and removes full screen control
from video alpha display UI.
Adds jasmine test for subtitles scroll.
Adds tests for caption change height functionality.
Uses gettext wrapper.
Fixes caption change height functionality.
Removes IS_CMS, isCms flags.

Fixes HD button
Disables Fullscreen mode in the Studio
Fixes ui controls bug

Initialize on every page that has a context_course set

https://docs.djangoproject.com/en/dev/ref/settings/#std:setting-ADMINS
https://docs.djangoproject.com/en/dev/howto/error-reporting/

not common.  This ensures that we load the MITX_FEATURES
token BEFORE deciding to disable CSRF.