[Redesign] Remove gradient headers

Summary: Ref T8099, this removes the gradient headers and their edge cases in Pinboard View, Setup Issues, Exceptions, and Document Heiriarchy.

Test Plan:
Tested each of these, grep for "gradient" and verify all states are gone.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D12944