R1:2bbe3b0cbfce

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…