Implement a projects() datasource function

Summary:
Ref T4100. This is like members(), but is implemented on top of the raw datasource. This is a lot simpler and involves way less code duplication.

I'll go back and implement members() like this, too.

Nothing actually uses this yet.

Test Plan:

• Used browse view to browse datasource.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12458