Summary:
Ref T2009. These classes are "Differential" now, but are used elsewhere in diff infrastructure (e.g., Diffusion).
- Rename them to "PHUIDiff".
- Move them to "src/infrastructure/".
- Give them a base class.
Test Plan: Interacted with inlines in unified and side-by-side views.
Reviewers: btrahan
Subscribers: epriestley
Maniphest Tasks: T2009
Differential Revision: https://secure.phabricator.com/D11996