give proper filenames on download

[zooko]

As reported by Jonathan Tapicer:

The file download url should send a header with the correct file so the
browser shows it for saving, sending the 'filename' parameter to the Tahoe
node via GET seems to have no effect, the filename is always a long
incoherent string of characters. For example, try this link:
http://tahoebs1.allmydata.com:8011/uri/URI%3ACHK%3A7u9ffi6gzsoi7qzj55783qu9k
w%3Adb9y3ep7n1s3nui3h1bk34riqcrk4xjtowjo57nikdfpzo8ojamy%3A3%3A10%3A68560?fi
lename=foo.txt&save=true

According to Brian and/or RobK, the only way to really get the browser to give the right filename is to append "/foo.txt" to the end of the URL, and teach the tahoe backend (webish.py) to ignore that trailing filename-shaped thing.

(Note: I'm leaving Component: unset for the moment in order to test a fix to the ​uncategorized tickets report.