mwconf: Enable $wgDBerrorLog and $wgRateLimitLog

Also:
* Factor out directory logic into $wmgJobWorkspace and $wmgMwLogDir.
* Remove comment about the Jenkins job having to create and make writable
  each of the individual files used here. As of I8c892abc46ed57f4a88 in
  jenkins-job-builder-config, the log/ directory itself is made writable
  instead so we don't have to do that anymore.

Change-Id: Icd2b25fd882b7953ee187b34fdc07fd1cc6715bd