Summary: This is the backend half of uploading an image as a header for Phame Blogs. Allows you to upload image, or delete it. Ref T10901
Test Plan:
Go to Manage Blog, visit Edit Header Image, Upload snarky file. See snarky file on Manage page. Edit Header Image, click delete, save, see file goes away.
{F1690966}
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T10901
Differential Revision: https://secure.phabricator.com/D16140
