R1:14fe941c349c

Reduce the cost of generating default user profile images

Summary:
See PHI413. You can pre-generate these with `bin/people profileimage --all`, but they're needlessly expensive to generate.

Streamline the workflow and cache some of the cacheable parts to reduce the generation cost.

Test Plan:
- Ran `bin/people profileimage --all` and saw cost drop from {nav 15.801s > 4.839s}.
- Set `defaultProfileImagePHID` to `NULL` in `phabricator_user.user` and purged caches with `bin/cache purge --all`.