Re-enabling MW1.19 compatibility

Many users can not update to 1.20 yet because other extensions don't support it yet. Lets keep MW1.19 compatibility for a while.
The WikiPage class existed in 1.19 already. The only problem is the first author, which can be fixed with an if.

Change-Id: Id89d434d0d7dfca2173959bfb6be40548aff9907