Mark UIExamples as a prototype

Summary: Ref T9103. This application is only useful for developing Phabricator, and in general is not kept "production ready". Mark it as a prototype.

Test Plan: visit /applications/, see it marked as prototype.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9103

Differential Revision: https://secure.phabricator.com/D13822