[Redesign] Better legibility on ObjectItemLists

Summary: Ref T8099, slightly lighter font on titles, darker greys on bylines, icons

Test Plan: Tested item lists in Maniphest, Differential, Audit

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D12919