Manual:$wgAjaxWatch/en

Details
Enable watching/unwatching pages using Ajax. Requires  and   to be true too. Causes wfAjaxWatch to be added to.

In MediaWiki 1.27, $wgAjaxWatch has been removed. The functionality is now always enabled.

Other
If you want to add a Watch/Unwatch tab (the tab with the star), set  to true.