Use a lighter color for completed list items in remarkup

Summary: In remarkup lists, it can be hard to clearly see which items still need to be completed. This makes completed items a little lighter for clarity.

Test Plan:
Review a long list with checked and unchecked items in a task.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17828