FSFile and TempFSFile cleanups

FSFile and TempFSFile cleanups

* Remove wf* function dependencies. This includes wfTempDir().
  Callers now should specify the directory, though it will try to do
  most of the wfTempDir() logic anyway if they do not.
* Update callers to inject wfTempDir() so $wgTmpDirectory is used by
  TempFSFile instead of it probing to find a valid directory itself.
* Move most of the wfTempDir() logic to TempFSFile::getUsableTempDirectory().
* Remove unused getMimeType() method.

Change-Id: Idd55936b07f9448a6c90577708722b7b52b8fe66