Gallery improvements

I’ve set up alternative views of all of the galleries. All the previous options are still there, so it’s still possible to filter the galleries by species, country or date. The main change is that it’s now possible to choose the layout. Previously the gallery displayed in a grid of thumbnails of three columns (at least on screens large enough to display it that way). This option remains and is now labelled as ‘small’. The new layout displays those images much larger in a single column occupying the full width of the area, and is labelled ‘large’.