Add support for `hg push --publish`
This should be simple enough to display/hide a button based on whether or not the
topic extension is enabled. I'm thinking add a button next to the existing push icon (icons/24x24/actions/hg-push.png and icons/scalable/actions/hg-push.svg) so that it's more convenient than going into the sync widget. My idea is to use the existing icon and add a checkered finish flag to it similar to https://www.pngegg.com/en/png-piwjf, but I'm open to other ideas.