mediawiki.page.watch.ajax: Disable watch link while waiting for API response

Previously clicking the watch link several times in a row would fire
several requests to the API, and the final result would depend on the
order in which they arrived. Let's not allow that.

This also removes the special-case for the 'loading' class, it is now
always added or removed.

Change-Id: Ib91ec8505d04945dc77e48ab70b5c94e3da47d9c
Merge "mediawiki.page.watch.ajax: Disable watch link while waiting for API response"