TNL 2098

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

- Remove grader(s) upon deletion of existing entrance exam
- Remove grader(s) upon addition of new entrance exam

Support iOS receiving the update even when in the background.
Support Android detecting duplicates.

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.

- sharing enabled for custom URLs only if URL actually specified

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

As some i18n features need ```ugettext``` or its shorcut ```_``` to be called on a non-string-literal, so the pylint check of ```translation-of-non-string``` should be explicitly disabled in such situations.