Added back on hover CC button functionality.

<b>Description</b>
When https://github.com/edx/edx-platform/pull/1183 was merged, somehow the functionality:

- on mouse over CC button, auto show transcripts if they are hidden; hide them again once the mouse leaves the CC button or transcripts

was lost. This PR enables it.

<b>Reviewers</b>
- @jmclaus