Migrate to using softload_json for loading / error handling json from disk.
Merge branch 'develop' of github.com:learningequality/ka-lite into 590

Conflicts:
	kalite/i18n/management/commands/generate_subtitle_map.py
Don't reshow download button when it's disabled.
Fix bug with use_local flag.
Merge branch 'develop' of github.com:learningequality/ka-lite into develop
Merge branch '590' of github.com:bcipolli/ka-lite into 590
Merge pull request #1295 from bcipolli/590

Show download size, potential disk space saving, from video download page.
Merge branch 'develop' of github.com:learningequality/ka-lite into translation_scrub