[Redsign] Update Hovercards for redesign

Summary: Ref T8099, updates hovercard headers, spacing, and borders.

Test Plan: Hover.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13365