Add "final" to all Phabricator "Controller" classes

Summary:
These are all unambiguously unextensible. Issues I hit:

• Maniphest Change/Diff controllers, just consolidated them.
• Some search controllers incorrectly extend from "Search" but should extend from "SearchBase". This has no runtime effects.
• D1836 introduced a closure, which we don't handle correctly (somewhat on purpose; we target PHP 5.2). See T962.

Test Plan: Ran "testEverythingImplemented" unit test to identify classes extending from final classes. Resolved issues.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T795

Differential Revision: https://secure.phabricator.com/D1843