07c5850b changed the widget used for the "mark as read"
menu item from OO.ui.ButtonOptionWidget to mw.echo.ui.MenuItemWidget.
While this makes sense, it also broke its functionality, because
mw.echo.ui.MenuItemWidgets are unselectable so if you click them
the 'choose' event isn't emitted.
However, we also can't make these widgets selectable, because
that breaks the links inside these widgets (the containing MenuSelectWidget
intercepts mouse events and prevents default). So instead,
make mw.echo.ui.MenuItemWidgets only be unselectable if they are
links (have config.url set), so that both use cases work.
Bug: T128432
Change-Id: I8374e3ae61a005ec7d25c2eb95b1391cbb191d60
dc48dfa022