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 15.801s → 4.839s.
• Set defaultProfileImagePHID to NULL in phabricator_user.user and purged caches with bin/cache purge --all.
• Loaded user directory.
• Saw default images regenerate relatively quickly.

Differential Revision: https://secure.phabricator.com/D19168