Render Conpherence threads as a separate menu inside the layout, not a side nav

Summary: Currently, the thread list is in a standard side nav, but that makes it awkward to render (rendering logic needs to live in the base controller) and gives it some bad beahviors (like autohiding on mobile). Instead, move it into its own view and make it a little more custom. Ref T2421.

Test Plan: {F38098}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T2421

Differential Revision: https://secure.phabricator.com/D5504