Wrapping up image swapping script for high res feature tile images.

Resolves #3267

@DFurnes @sbsmith86 

Dave, took a slightly different approach than needing to remove the data role after image swap. Instead, saving all feature tiles to an array; if array empty, then images swapped, otherwise they've already been swapped and added to the "completed" array and thus no action required :)

All changed `$galleries` var to `$mosaicGalleries` so it's more descriptive of the actual jQuery collection obtained.

CC: @aaronschachter @barryclark