Add tiled grid options to Overviews and Galleries

[hugopeek]

Could use Masonry / Isotope in later iterations, but the primary goal is to display image grids without a gutter between the items.

I already started experimenting on Person and Project overviews by adding a Tiled template variant, but I can't recall if that got implemented in core..

Main challenge is to decide how to include the images. CSS background images offer more design flexibility, but backend implementation is a bit hacky. An alternative could be to use the object-fit technique on regular <img> tags. That way, things like 'background-size: contain' can be used on the image tag as well, but it will require a polyfill for IE.

Also consider the upcoming implementation of responsive images: hugopeek/romanesco-patterns#71

[hugopeek]

Link to object-fit technique: https://css-tricks.com/almanac/properties/o/object-fit/