fix #1242: check first if subtitles dir exists
Merge pull request #1245 from aronasorman/1242

fix #1242: check first if subtitles dir exists
setup english language on startup
installed_language_packs api now uses session['language_choices']
add api endpoint for refreshing language packs
refresh language pack list after language pack download is done
also refresh language choices when we first load update_languages
Merge pull request #1249 from aronasorman/1244

Add "en" LanguagePack on server startup, use cache of installed language choices
Adding packagees, so PR #6 can be clearer
Merge branch 'develop' of github.com:learningequality/phase-2 into local_content_tools

Conflicts:
	kalite/shared/i18n.py