[SCHEMA] Minimal contact search implementation.

Summary:
Provide a simple per-user contact search ability. Currently, it only uses a
simple prefix-completion heuristic, and doesn't rank results other than
alphabetically, but it's fast and works.

Depends on D38.

Test Plan: Unit tests added.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D40