Run jsduck through grunt

* New grunt commands: jsduck, docs
* Replace ruby jsduck with grunt jsduck task

If you're not running jsduck from inside vagrant then you can pass
the MediaWiki install path as an argument so that jsduck is able to find
the Mantle javascript files:
* grunt docs --MW_INSTALL_PATH=/some/path/
* make jsduck MW_INSTALL_PATH=/some/path/

Bug: 72718
Change-Id: I2ebcf50680e6d4a7e37a4dfb4247279385c0bcd9
Merge "Run jsduck through grunt"