Rough in the new custom URI panel

Summary: Ref T10748. Ref T10366. No support for editing and no impact on the UI, but get some of the basics in place.

Test Plan: {F1223279}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10366, T10748

Differential Revision: https://secure.phabricator.com/D15742