Update project profile image composer for new IconSet code

Summary:
Fixes T6856. Fixes T10164.

• Make the profile image composer code use the underlying icon name instead of the top-level icon key, so it works instead of 404'ing.
• Change the button to show a preview of the profile icon instead of the text "Use Icon and Color".
• When creating a new non-milestone project, automatically set the profile image to the icon + color image.

Test Plan:

• Created several new projects, saw appropriate default icons.
• Edited projects, saw icon previews.
• Clicked icon buttons to set icons.
• Poked around other applications which use builtins (Pholio, user profiles) to look for anything I broke, but everything seemed fine.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6856, T10164

Differential Revision: https://secure.phabricator.com/D15050