Clean up list styles in Remarkup

Summary: Fixes T8643, makes lists with and without checkboxes align and display better.

Test Plan:
Lists with and without checkboxes.

{F1111426}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T8643

Differential Revision: https://secure.phabricator.com/D15272