Implement open() and closed() Maniphest tokenizer status functions, plus cleanup

Summary:
Ref T4100.

• Make it easy to choose all open or closed tasks.
• Make "special" tokenizers composable.
• Get viewer() generating documentation properly.

Test Plan:

• Ran queries with new tokens.
• Browsed new tokens.
• Viewed docs on new tokens.
• Used plain status tokens.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12530