Create script to record video file sizes
Generalizing retrieval of metadata
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
Migrate to using softload_json for loading / error handling json from disk.
Show (actual) local and (estimated) remote video sizes.
Merge branch 'develop' of github.com:learningequality/ka-lite into 590

Conflicts:
	kalite/i18n/management/commands/generate_subtitle_map.py
fixing up code refactoring, defining assert
Fix bug in tabular coach reports.
Don't reshow download button when it's disabled.
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.