Use current content model for blank page content (not title default)

Use current content model for blank page content (not title default)

This means that if you specify ?model=MODEL_ID you can create a
page using the non-default content model (but only if you have
the correct rights. Admin by default). Previously this only worked
if you specified a preload parameter that had the content model
that you wanted.

Additionally, I believe that this makes more sense logically in
terms of using the class state instead of the global state for
the current content model.

Change-Id: I938be1abcae8ac0b5fdb82d0d57bad5526aa5dee