GlotPress grunt task

This implements a grunt task to download .mo files from our GlotPress install. 

The minimum required percentage is 50% translated, it doesn't take waiting strings and it only downloads .mo files. Also added a commit that updates a BUNCH of .mo files to show the impact of this. 

I've also deleted bg_BG, bs_BA and ca as they were under 50%, this is done manually, not by the Grunt task yet.

@tacoverdo please review :)

**note** to @tacoverdo and self: zh is not a valid locale. We'll need to change it to zh-cn for _all_ projects in the yoast translate database.