Create script to record video file sizes
Generalizing retrieval of metadata
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.
Tweak new messages for punctuation, capitalization, and to be less fragmentary.
Small translation tweaks, based on po file scrubbing.
Merge branch 'translation_scrub' of github.com:bcipolli/ka-lite into translation_scrub