Move token rendering into Datasources

Summary:
Ref T4100. I want to reduce the amount of code duplication that function datasources currently need to wrap some parameter datasource.

For example, ProjectMembersDatasource should really just be a little bit of logic on top of ProjectsDatasource, which should do most of the heavy lifting.

Moving rendering into datasources brings us a step closer to being able to do this.

Test Plan:

• Rendered normal, function, and invalid tokens.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12457