Extract mediawiki::settings from mediawiki::extension.

Extraction refactoring of LocalSettings configuration file generation logic
from the mediawiki::extension resource type. Allows use of settings without
requiring an extension to be involved.

This will come in handy for things like thumb.php support which only need to
introduce configuration changes.

Change-Id: Icf6f228de5083337306d3fde935c03e3ee39630f