Implement viewer() and members(project) typeahead functions

Summary:
Ref T4100. This is still a bit rough around the edges, but mostly does what we're after.

• Implements viewer() and members(...) functions.
• The new browse workflow makes these discoverable.

Test Plan: {F374201}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12444