Add basic support for Phame blog headers

Summary: Fixes T10901. Allows blogs to have headers. I've built this in a basic way, any file, max-height is 240. Should bleed into top crumbs, so any spacing you want you should add to the file itself. Might have to see how users break this.

Test Plan: Set a blog header, see blog header, remove blog header, see no blog header. Check mobile, tablet, desktop break points.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10901

Differential Revision: https://secure.phabricator.com/D16141