mediawiki.api.watch: Don't use deprecated 'title' parameter

* Instead handle it client side, and not as a deprecated feature.
* Rename 'page' parameter to 'pages'.
* Add unit tests.

Change-Id: I81ec4282aedd91e3f0bb2ed0d6dd34e437b54f5d
Merge "mediawiki.api.watch: Don't use deprecated 'title' parameter"