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

Filter by Language:
Designation for course language(s) in Studio
allows for filtering of courses based on their language in the sidebar
Reuses active filter view UI/IA from find courses page on edx.org
view for clearing individual or all filters, as with edx.org filtering

Conflicts:
	cms/djangoapps/contentstore/tests/test_courseware_index.py

Added setting to enable/disable library indexing + moved is_index_enabled into search indexer classes
Added task and signal to fire library reindex

Search indexer is not bound to any library modifications, so tests fail - this is expected.

1. Fix the broken `gettext()` call in `courde_detail.js`
2. Change the string to avoid Transifex misunderstand the pattern '% o' in `checklist.underscore`.
3. Embed the `{filename}` part inside the string in `transcripts-choose.underscore` to provide better i18n flexibililty.
4. Remove all the call of `gettext()` and `${_()}` inside the `mock` directory to aovid mis-extraction by pybabel. (It seems that the `mock` directory is soly used in test? So the strings in this directory need not to be extracted and translated?)
5. The unicode char `\u2014` will trigger a bug inside pybabel which makes the extracted string untranslatable, so remove this char.

Indexing moved to celery task. Reorganised indexing to always allow for full course reindex. Indexing code no longer needs to be in common, because it is triggered by signal instead of being called.g