Things done with this patch:

1) Changed the 'Use photo frame' option from being a plugin variable
   to being an instance config variable

2) Added back in the css for the giving the gallery images photo frame
   borders

3) Get the non-square thumbs to render correctly spaced on load

4) Swapped the slimbox2 to fancybox3 and allowed the block itself to
control when to put content into head data

behatnotneeded

Change-Id: I397be45f2347588988b663b25b560d8ab876afe1
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>