When a project has a custom icon, use that icon to label the project policy in the policy dropown

Summary:
Fixes T8808. Currently, all project use the default ("Briefcase") project icon when they appear in a policy dropdown.

Since project policies are separated out into a "Members of Projects" section of the dropdown anyway, there is no reason not to use the actual project icon, which is often more clear.

Test Plan: {F6849927}

Maniphest Tasks: T8808

Differential Revision: https://secure.phabricator.com/D20799