Actually return contact search results highest-scoring first.

Summary:
Previously we were (1) returning results lowest-scoring first, and (2)
limiting the result count before ranking, instead of after.

Test Plan:
Still needs a regression test, but for now manually verified via
`zerorpc tcp://0.0.0.0:9999 search_contacts <account_id> <search_string>.`

Reviewers: spang, mg

Reviewed By: mg

Differential Revision: https://review.inboxapp.com/D111