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
Merge branch 'develop' of github.com:learningequality/ka-lite into translation_scrub
Don't reshow download button when it's disabled.
Export csv form and datepicker logic and css.
Refactor video utils into more generic utils.internet.download
Add the progress bar; get callbacks about % downloaded along the way.
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
Merge pull request #1296 from bcipolli/translation_scrub

Scrub strings for translation
amending to fix bug in showing the download video button
small styling tweak to vertically space things out
Merge branch 'develop' of github.com:learningequality/ka-lite into 1141
add --no-kalite-trans option
add gc collect calls to remove lingering garbage from removing
nonmetadata from po files
add extra msgblock removing logic into polib.py itself
solve issue #1289: use POFile.merge for concatenating files instead of msgcat