Fix changedesc captions field for screenshot objects.

The new captions code for changedescs was trying to access a 'filename'
property on Screenshot objects, but they didn't define such a property. This
change adds it.

Testing done:
Viewed /r/822/ from r.rb.o and saw a usable page with a change description that
included the caption, rather than a traceback.

Reviewed at https://reviews.reviewboard.org/r/5525/