Detect and prevent invalid configuation of "ui.footer-items"

Summary: Fixes T12775. Currently, we do not validate this option and it's possible to configure it in an invalid way.

Test Plan: Tried to misconfigure things, was helpfully pointed toward errors.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12775

Differential Revision: https://secure.phabricator.com/D18041