Adjust alert dismiss button markup/style

Use a button element for the dismiss button instead of a span.  Also,
position the dismiss button to the far right of the alert bar instead of
just to the right of the text.

Also remove the 1px white border from the builder header.

Addresses #548
Update required version of django-filer

Since the blocking issue is no longer relevant, use the stable version
from pypi instead of the development version on GitHub.