Exclude lonely question marks from the linkify regex

Many URLs we see in the KDE rooms end with a question mark, without a
space. The linkify regex for plain URLs incorrectly considered them as
part of the link, which usually breaks them when opened in a web
browser. Now the regex excludes these, unless they are accompanied by
another character (so links like kde.org/realurl?is=true will still
work.)

(cherry picked from commit 1b7f482d)