Update to allow stepwise show more functionality.

Fully Resolves #3265

Fixes the Revealer so that if a mosaic gallery has more than 5 items, instead of revealing all after clicking the "Show More" button, it will reveal 2 rows at a time until it runs out of rows (wink wink @mikefantini).

@DFurnes 

CC: @barryclark @sbsmith86