🎁 WIP - Adding Thumbnail Generator skeleton

[jeremyf]

This commit introduces the skeleton of the thumbnail generation necessary for PDFs, images, and the images that we make from the PDFs.

It does not have the full business logic to perform this work; but the specs are mostly in place and can then be used to drive development.

Related to #22

• https://github.com/scientist-softserv/derivative_rodeo/issues/22

[jeremyf]

@kirkkwang I've added you as the review as I want you to then work on #22