Please add live previews of thumbnail options

[diamondsw]

Is your feature request related to a problem? Please describe.
There are numerous thumbnail options (engine, size, fit, quality), and right now (first install) I have no idea what these will look like. I'd rather know before I throw thousands of images at this.

Describe the solution you'd like
It would be extremely helpful if there was a live preview of what these look like to make choosing a favorite easier. Include a sample SFW image and as options are changed, update the thumbnail on the fly.

Describe alternatives you've considered
I could just YOLO and add images to my booru and then change options, but then I have to figure out how to regenerate thumbnails en mass, and it all gets to be a pain. Especially for a first-time user.

[diamondsw]

Also, when thumbnail sizes are changed, the spacing of the layout (at least on the Danbooru2 theme) does not adjust for the larger thumbnails. Seems only 192x192 has been tested and is hardcoded in. Added this as a new feature request in #938.