1) Introduce EntryPointEnvironment which wraps functions that interact
with the PHP runtime, so they can be mocked for testing.
2) Allow server info fields to be overwritten in FauxRequest.
3) Make MediaWikiEntryPoint use WebResponse to set headers
Bug: T354216
Change-Id: Ic21950c956de5d2b5a7dd66a1e2de58f807cfd9f
