Disallow `[1]` from being interpreted as a marked checkbox in remarkup

Summary: Fixes T6640. `[1]` and similar are more likely footnotes than checkmarks.

Test Plan: {F241922}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6640

Differential Revision: https://secure.phabricator.com/D10913