First pass at a custom url shortener for loom.io

This allows users to put in an up-to-15 char custom code that'll redirect  to their group / discussion, so for example I can put in `kitty_pics` as the short_url for my group / discussion, and `http://loom.io/kitty_pics` (or `www.loomio.org/kitty_pics`) will redirect there.

- Still some wonkiness for error message display on forms
- Need to circle back and rock some more validation on the user input, so they don't input slashes, special chars, and other garbage. It does throw a validation error if a route is already present in the app for something else (like `/groups`)
- Not stoked about the way we handle deleting urls that have been updated to blank (we save the record, then delete if it's nil or blank), but seems like an edgy case.