Rename "DifferentialChangesetViewManager" to "DiffChangeset"

Summary: Ref T12616. This class is already mostly-reasonable as a representation of an individual changeset, so I plan to just adjust it a little bit.

Test Plan:

• Used git grep to search for ChangesetViewManager.
• Used git grep to search for changeset-view-manager.
• Browsed around and interacted with changesets.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17842