htmlscrubber: Allow colons in url fragments after '?'

htmlscrubber: Allow colons in url fragments after '?'

Colons are not allowed at the start of urls, because it can be interpreted
as a protocol, and allowing arbitrary protocols can be unsafe
(CVE-2008-0809). However, this check was too restrictive, not allowing
use of eg, "video.ogv?t=0:03:00/0:04:00" to seek to a given place in a
video, or "somecgi?foo=bar:baz" to pass parameters with colons.

It's still not allowed to have a filename with a colon in it (ie
"foo:bar.png") -- to link to such a file, a fully qualified url must be
used.