Add support for letter-/pillar-box cropping of browse category images

[corylown]

This comment explains the issues we were having with Virtual Tribunals (VT) and image cropping in the context of browse category tiles.

VT has a lot of browse categories, and relatively few images, as our collections are text-heavy/centric. When we have the opportunity to use images from a collection or find public domain images, we often have to crop them to display a certain area. In this use case, I am discussing working with images as Browse category tiles.

Steps we took:

1. Chose an image with a specific detail we wanted to feature (head and shoulders of a defendant)
2. Went to the browse category section in exhibit dashboard, added the image and wasn’t able to fit the head and shoulders part of the photo within the parameters of the cropping tool without cutting off the person’s forehead.
3. Because of the parameters not fitting, we then used Gimp (open source Photoshop) to edit, located the aspect ratio required to appear correctly in a browse category tile, and cropped the image in the Gimp application instead, using letterbox "bars" above and below.
4. The image was then saved as a JPEG and re-uploaded as a Browse category tile successfully.

Expected behavior / would like to see:

1. We would like to be able to have a background image letterboxed-style as a part of the cropping feature, which would make the task of uploading images to the Spotlight browse category section more fluid and allow us to skip the Gimp / photo editing steps outside of the Spotlight application.

Let me know if any more detail is needed for this user story.

Originally posted by @laurensorensen in #357 (comment)