Support audio files with HTML5 `<audio />`

Summary: Ref T3887. Similar to how we render images with `<img />`, render audio with `<audio />` if possible.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3887

Differential Revision: https://secure.phabricator.com/D7156