Choose default project image by icon

Summary: Builds out a map for icon->image in Projects, selects the icon's image as the default project image if there is no custom image chosen by the user.

Test Plan: Select various icons, see image change. Test choose picture, pick a new image.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18174