Taxonomy gallery show more

Addresses the "greater than 5 campaigns" show more issue in ##3265. This provides a new JS object where you can pass a container element, a collection of 1 or more items inside container and it will hide them, and then show them via a "Show More" button. 

This also will work smoothly if there happens to be more than one mosaic gallery on a page and hide extra items on all of the galleries. @mikefantini said it's highly unlikely, something like 0.00001%, but that means there's still A CHANCE!

@DFurnes 

CC: @barryclark @aaronschachter @sbsmith86