Allow Remarkup lists to have checkboxes

Summary:
Fixes T3945. This provides a convenience style for checkmark lists. They're read-only. See disucssion in T3945.

We're limited on how much we can style the checkboxes -- it would be nice to make them a little more visible, but this seems mostly OK to me. I added some hinting to the item text to make the completion state more clear.

Test Plan:
  - Added unit tests.
  - Ran unit tests.
  - Fixed a couple of existing unit test issues.

{F149711}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3945

Differential Revision: https://secure.phabricator.com/D8931