Fix comment previews in Quicksand

Summary:
Fixes T7063. Fixes T7059. We were running some extra unnecessary behaviors. In particular, we would render the menu but not actually send it over the wire, so some behaviors would execute, fail to find the nodes they expected, and throw. This cascaded into some other failures.

Do less work and don't activate behaviors which won't be able to run.

Test Plan: Clicked around, no more errors on page transition. Comment previews work.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7059, T7063

Differential Revision: https://secure.phabricator.com/D12188