Modernize "users" typeahead datasource

Summary: Ref T4420. Modernize users.

Test Plan:
- Edited "Commit Authors" on Audit search.
- Edited "Created By" on calendar search.
- Edited "invited" on calendar search.
- Edited "To" on "New conpherence message".
- Edited user on "Add user to conpherence thread".
- Edited "Authors" on countdown search.
- Edited "Author" on differential search.
- Edited "Responsible users" on differential search.
- Edited "Owner" on Diffusion lint search.
- Edited "include users" on Feed search.
- Edited "Authors" on file search.
- Edited "Authors" on Herald rule search.
- Edited a couple of user-selecting Herald fields on rules.
- Edited "user" on legalpad signature exemption.
- Edited "creator" on legalpad search.
- Edited "contributors" on legalpad search.
- Edited "signers" on legalpad signature search.
- Edited "Authors" on macro search.
- Edited "Reassign/claim" on task detail.
- Edited "assigned to" on task edit.
- Edited "assigned to", "users projects", "authors" on task search.
- Edited "creators" on oauthserver.
- Edited "authors" on paste search.
- Edited "actors" and "users" on activity log search.
- Edited "authors" on pholio search.
- Edited "users" on phrequent search.
- Edited "authors", "answered by" on Ponder search.
- Edited "add members" on project membership editor.
- Edited "members" on project search.
- Edited "pushers" on releeph product edit.
- Edited "requestors" on releeph request search.
- Edited "pushers" on diffusion push log.
- Edited "authors", "owners", "subscribers" on global search.
- Edited "authors" on slowvote search.
- Edited users in custom policy.
- Grepped for "common/authors", no hits.
- Grepped for "common/users", no (relevant) hits.
- Grepped for "common/accounts", no (relevant) hits.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9885