Options to hide and show download captions and transcript buttons

Releases #19.

Add transcripts download and refactor captions download

Releases #11

Show download transcript button if no access token present

• Release changes from #9.
• Upgrade Django version in CI/dev bundle

Add Download Transcripts button

This is the first tagged release. It includes latest updates from #6.

Final Python 2 release Ironwood and prior compatibility

Merge pull request #4 from appsembler/dependabot/pip/django-1.11.29

Bump django from 1.11.28 to 1.11.29