Disabled Uninstall state for essential applications

Summary: Disabled Uninstall state for essential applications of Phabricator. Information provided why they cannot uninstall these applications. Also take care of users trying to install an application which cannot be uninstalled by editing the URI.

Test Plan: Manually tested

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4743