Drive differential revision list with custom fields

Summary:
Build the revision list table out of custom fields instead of hard-coding it, so
installs can add all sorts of zany things to it.

This implementation will preserve the expected behavior:

public function sortFieldsForRevisionList(array $fields) {
  $default = new DifferentialDefaultFieldSelector();
  return $default->sortFieldsForRevisionList($fields);
}

Test Plan:

• Loaded differential revision list, identical to old list.
• Profiled page to verify the cost increase isn't significant (it's quite

small).

Reviewers: jungejason, btrahan

Reviewed By: btrahan

CC: aran, btrahan, davidreuss, epriestley

Maniphest Tasks: T773, T729

Differential Revision: https://secure.phabricator.com/D1388