PLAT-345 ensure sender object is passed to doEdit for correct welcome message attribution

It appears that the sender object has been missing from the parameter list to Article::doEdit which is responsible for the work of creating the welcome message. As a result, the `$wgUser` is being used as the default author in a batch context which appears to resolve to localhost or 127.0.0.1 in this context.

This pull request adds the sender to the `doEdit` calls and adds some testing to ensure that the user object is part of the parameter list. 

I've also attempted to make the testing more robust by stubbing some of the functionality that results in IO. I specifically wrapped a few of the calls to `wfMessage` to prevent database access in the unit tests.

/cc @Wikia/platform-team @michalroszka