Provide a "PHUIFormIconSetControl"

Summary:
Ref T9992. This is a step on the path to getting EditEngine working in Badges, Projects and Calendar.

This doesn't add a new EditField for icons yet, just standardizes the old stuff. New stuff is more general and I saved 150 lines of code.

I put the endpoint in Files because the similar "choose a profile picture" endpoint will definitely go there, and this endpoint might eventually feature, like, "draw your own icon~~" or something.

Test Plan:

• Created events, projects and badges with custom icons.
• Edited events, projects and badges, changing their icons.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9992

Differential Revision: https://secure.phabricator.com/D14799