Previously, captions were escaped twice: once with escapejs in the

reviewable_page_data.js template and again via the underscore template

in the RB.FileAttachmentThumbnail. Since the only place a

RB.FileAttachmentModel will retrieve an escaped value for the caption

is from the JS template, we unescape it there so that the model recieves

the correct value.
The filename is now also correctly parsed.

Ran JS tests.
Verified that a caption with unsafe characters was correctly set in

both the JS model and the database model.