Consolidate Conpherence frame rendering

Summary: Conpherence frame rendering is spread out a bit across ConpherenceController, ConpherenceListController, and previously ConpherenceNewController. I want to let the Thread and Widget controllers render the frame for mobile. Ref T2421.

Test Plan: Viewed conpherences, switched between conpherences.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2421

Differential Revision: https://secure.phabricator.com/D5501