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
