Add a profileimage generation workflow for the cli

Summary: Ref T10319. This adds a basic means of generating default profile images for users. You can generate them for everyone, a group of users, or force updates. This only generated images and stores them in files. It does not assign them to users.

Test Plan:
bin/people profileimage --all to generate all images.
bin/people profileimage --users chad to generate a user.
bin/people profileimage --all --force to force rebuilding all images.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10319

Differential Revision: https://secure.phabricator.com/D17464