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:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100, T5595

Differential Revision: https://secure.phabricator.com/D12462