Provide PhabricatorEnv::isSelfURI to test if a URI points at the current install

Summary:
Ref T5378. This repackages an existing check to see if a URI is a URI for the current install into a more reasonable form.

In an upcoming change, I'll use this new check to test whether http://example.whatever.com/T123 is a link to a task on the current install or not.

Test Plan: This stuff has good test coverage already; added some more.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5378

Differential Revision: https://secure.phabricator.com/D17502