findUrl: be pickier about what can precede a URL

findUrl() was seeing strings like "You have 1 new message in
foo@example.com/Inbox" and finding the URL
"[http://]example.com/Inbox". Require that URLs either start at the
start of the string, or are preceded by whitespace or an open
paren/quote/etc.

(Since JS doesn't have look-behind assertions like perl does, we have
to actually match the URL-preceding character in the regex, and then
adjust the result findUrl returns accordingly.)

https://bugzilla.gnome.org/show_bug.cgi?id=636252