R1:8ed48a89f451

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

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