[Redesign] Clean up AphrontDialog

Summary: Ref T8099, Cleans up UI issues, adds appendList and renders lists and paragraphs with Remarkup UI.

Test Plan: Test Policy Dialogs, other various dialogs.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13463