Simplify output URLs.

* Including the original filename in the output URLs just makes things
more complicated with little benefit, so we're leaving it out. Note
that the original "input" file still includes the filename in its URL.

* Use a 16 character hex string instead of the much longer UUID. This
should still allow plenty of unique variations.

* No longer use `uploads` path prefix which serves no real purpose.

This is all a pre-cursor to exposing the URLs to the end-user.
Expose output file URLs to end-user.

We've decided that we want the user to "own" their own asset URLs. So
we're no longer using our web app to redirect "internal" URLs to AWS S3
URLs.