Add mediawiki module for fetch and checkout hooks

To generate the localization cache, this change adds a fetch hook
and a checkout hook that will utilize the shadow reference
repository to generate the localization cache during the fetch
stage, then will rsync the cache during the checkout stage.

Assuming no files need to be changed, this approach should work
similarly to how the cache in generated from the deployment server.

Change-Id: I3fc95902b1c7668f55448a873933db58fc8e0a0a