add v1.3 with Accessibility, audio/x-m4a and ...

[qqmyers]

Making a draft PR in prep for a v1.3 release. So far this includes:

• Accessibility enhancements from QDR: fixing issues reported by Deque Axe w.r.t. color contrast, lack of a main landmark, missing img alt tags, etc. Open issues remain in the Spreadsheet previewer (Spreadsheet viewer has accessibility issues #4) and providing video captions (Support video captions for accessibility #3).
• Added registration of the Audio Previewer for the audio/x-m4a mimetype
• Uses the latest platform-specific favicons from Dataverse