Add z-index to action menu in PHUIDocumentView

Summary: Fixes T7822. Adds a z-index and cleans up menu padding.

Test Plan: Review action menu on Phriction article with hovercard token.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7822

Differential Revision: https://secure.phabricator.com/D12408