role::mediawiki: clear up nested class hierarchy

Extract the individual mediawiki role classes from the role::mediawiki { .. }
scope that enclosed them by explicitly qualifying each role.

To make this easier to review, I am not dedenting the code that moved up a
scope; I'll do that in a separate, formatting-only commit.

Change-Id: I9fe6495479efac4e77ea21e85814388c23285136