Consolidate changeset rendering logic

Summary:
Ref T5179. Currently, all the changeset rendering logic is in the "populate" behavior, and a lot of it comes in via configuration and is hard to get at.

Instead, surface an object which can control it, and which other behaviors can access more easily.

In particular, this allows us to add a "Load/Reload" item to the view options menu, which would previously have been very challenging.

Load/Reload isn't useful on its own, but is a step away from "Show whitespace as...", "Highlight as...", "Show tabtops as...", "View Unified", "View Side-By-Side", etc.

Test Plan:

• Viewed Differential.
• Viewed Diffusion.
• Viewed large changesets, clicked "Load".
• Used "Load" and "Reload" from view options menu.
• Loaded all changes in a large diff, verified "Load" and TOC clicks take precedence over other content loads.
• Played with content stability stuff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5179

Differential Revision: https://secure.phabricator.com/D9286