Made download filename more reliable (#1133)

Switched to using a whitelist approach where we only allow certain
characters and remove the others.  Switched from replacing characters
with "_" to simply removing them.  This is what django's
get_valid_filename() does and I think it will probably lead to nicer
looking filenames in general.
Merge pull request #1181 from pculture/gh-1133

Made download filename more reliable (#1133)