Fix bug-assist box position.

In commit https://dvcs.w3.org/hg/text-tracks/rev/1c6d33305c80 the
styling of the bug-assist box seems to have been removed by the
bug-assist.js script. This patch re-introduces the most important
styles.
Merge pull request #15 from silviapfeiffer/text-select-bug-report

Fix bug-assist box position.