Add a utility for converting Live Editor audio to a new format.

Summary:
This adds in the `audio_convert.html` utility for converting audio recordings to a new format (in this case, one based around the insertText/removeText commands in Ace).

Additionally a change was made to LiveEditor such that it would clean up after itself upon removal (more work still needs to be done, namely we need to remove tooltips, as well).

Test Plan: Run the utility, make sure it outputs usable JSON! (Let's hope!)

Reviewers: brian, pamela

Reviewed By: pamela

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D13257