Enable mediawiki-core-npm (runs jshint+jscs), disable mediawiki-core-jslint

For front-end tests, I've started migrating things to run with
Grunt. This makes it possible to easily run these locally, and
puts the logic for it in the repository (instead of duplicating
them somewhere in a blackhole in jenkins-slave-scripts).

This should help make things more consistent between what your
text editor plugin uses (local .jshintrc file etc.) and what
Jenkins will say after submitting a change.

It also starts making things run in labs for mediawiki-core
(previously done for oojs, and VisualEditor already), and starts
on running multiple build steps in pone job per change set
instead of one job for each build step (this one runs jshint and
jscs at the moment), and allows the local repository to extend
this without requiring changes to jjb or zuul.

*-qunit test and *-jsduck are still separate for now.

Change-Id: I8a7ccef26da45d9ed7c7705df77246001bb85544