Rename inline comment views to "PHUIDiff" and give them a base class

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