Add ability to set a header image per Phame blog

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