Add an "availaiblity" attachment for user.search

Summary: Ref T13222. See PHI990. The older user.query supports availability information, but it isn't currently available in a modern way. Make it available.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19851