Use a disk-based default avatar, not a database-based one

Summary:
This is mostly in an effort to simplify D2323. Currently, we load one image into the database by default. This is a weird special case that makes things more complicated than necessary.

Instead, use a disk-based default avatar.

Test Plan: Verified that a user without an image appears with the default avatar as a handle, in profile settings, and on their person page.

Reviewers: btrahan, vrana, edward, jungejason

Reviewed By: vrana

CC: aran

Maniphest Tasks: T345

Differential Revision: https://secure.phabricator.com/D2331