Allow the default Remarkup <p> tag to be inline-styled for mail

Summary: Ref T10694. Browsers / mail clients have default <p> rules, particularly margin/padding. This allows the inline stuff to turn them off with explicit "padding: 0; margin: 0;" so it can control spacing more precisely.

Test Plan: See D15857.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15858