Fix double escaping on file attachment captions.

In review UIs, the caption text was getting double escaped. The reason for this
was that we used a template to dump it into javascript, which would escape it
once, and then called $.text(), which would escape it a second time. The fix
here is to use the escapejs filter when templating the value into javascript.

Testing done:
Created a file attachment with a caption that had some quotes in it, and
verified that the visible header in the review UI didn't have the literal
"&quot;".

Reviewed at https://reviews.reviewboard.org/r/6448/