PLAT-345 wrap $wgUser object when posting to wall

Followup replacement pull for https://github.com/Wikia/app/pull/4336.

The call stack used by buildNewMessageAndPost does not pass the `$user`
object all the way down the stack but instead falls back to `$wgUser`. We
have two choices-- we can alter core to pass `$user` down or we can wrap
the global. The former is preferred but risky. The latter is a compromise with the
`GlobalStateWrapper`. Going with the latter.

/cc @owend @michalroszka