webcommands: move nonempty logic from JavaScript to Python
This avoids hard-coding the '(none)' string in the JavaScript for each
style and avoids the {nonexisting|nonempty} hack which only works
since the template system don't complain about undefined variables.
200 Script output follows
content-type: text/plain
content-length: 157
content-disposition: inline; filename="some \"text\".txt"
This is just some random text
that will go inside the file and take a few lines.
It is very boring to read, but computers don't
care about things like that.
host - - [date] "GET /?f=a23bf1310f6e;file=sub/some%20%22text%22.txt;style=raw HTTP/1.1" 200 -