Merge behavior-show-more into behavior-populate

Summary: Ref T2009. It doesn't make sense to have these as separate behaviors. We require a ChangesetViewManager to track view parameter state.

Test Plan: Interacted with changesets in Phriction, Differential and Diffusion.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11979