Add an Almanac service typeahead datasource

Summary: Ref T5833. Ref T6238. This will probably be useful somewhere in the upstream eventually, and is materially useful in the Instances application right now.

Test Plan: tippy typey typey

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6238, T5833

Differential Revision: https://secure.phabricator.com/D11016