Add ui/urls.h to define some URLs on various of our websites.  Use the
GitLab URL for the wiki.  Add a macro to generate wiki URLs.

Update wiki URLs in comments etc.

Use the #defined URL for the docs page in
WelcomePage::on_helpLabel_clicked; that removes the last user of
topic_online_url(), so get rid of it and swallow it up into
topic_action_url().