Modernize Audit search engine

Summary:
Fixes T9279. Modernizes the SearchEngine and Query classes. User-facing changes:

• Added order by commit date, default to order by commit date with newest commits first.
• Added explicit "Needs Audit by".
• Added new packages(...) typeahead function.
• Picked up automatic subscribers, projects, and order fields.

This changes behavior a little bit: we previously attempted to exclude, e.g., commits which a package you own needs to audit, but which you have resigned from. This is difficult in general and I think it needs a more comprehensive solution. This shouldn't impact users much, anyway.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9279

Differential Revision: https://secure.phabricator.com/D14013