Provide a url-shortening function

[dkapell]

It would be nice to be able to configure some convention level URLs manually, that didn't interact with site functions, to map CMS content like: https://u.interconlarp.org/pages/new-proposal to a top level path like https://u.interconlarp.org/propose, to make easier links to put in text/on posters/flyers/etc.

Idea from Julie Diewald.