Make "user's projects" a logical datasource and add viewerprojects()
Summary:
Ref T4100. Ref T5595. This converts projects(user) into a logical datasource.
Also adds a viewerprojects() logical datasource.
Test Plan: {F375546}
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T4100, T5595
Differential Revision: https://secure.phabricator.com/D12462