The {% thumbnail %} template filter has been around a long time, and

has remained pretty static in that time. It took in a FileField-backed

file instance, a WIDTHxHEIGHT string or (width, height) tuple (with

an optional height), and generated a thumbnail if one didn't exist.
It wasn't able to work with arbitrary File instances or file paths in

a storage backend, couldn't build an aspect ratio around just a height,

or tell us the path of a thumbnail without first generating it. And the

code was playing it fast and loose with input types.
This change modernizes much of this method. It's now type-safe, capable

of working with a range of input types representing files, and

sanity-checks the files and sizes for validity.
It can now cap to either a width or to a height, maintaining aspect

ratio.
It can be told not to create a thumbnail if it doesn't already exist,

helping to retrieve the URL to a thumbnail in order to, say, delete it,

without doing the work of uselessly creating one first.
Unit tests were added to check all thumbnailing functionality.

Unit tests passed.
Made use of the new create_if_missing and file input types

functionality in an in-progress change.