Add "thread" role and allow threads to render without thread lists

Summary: The actual layout on mobile is a bit silly since the thread ends up being like 5px tall for now, but it technically works. Ref T2644.

Test Plan: {F38106}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T2421

Differential Revision: https://secure.phabricator.com/D5506