Restore viewer() function to "Responsible Users" tokenizer in Differential

Summary:
Ref T10939. This makes the viewer() function work again. It retains its own meaning (viewer, plus all their projects and packages).

There's no exact-viewer() function; we could conceivably add one eventually if we need it.

Test Plan:

• Queried for viewer(), got the same results as querying by my own username.
• Browsed function in token browser.
• Reviewed autogenerated documentation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10939

Differential Revision: https://secure.phabricator.com/D15951