send http 400 bad request when headers aren't right

[simonLeary42]

• getting $_POST["foobar"] without checking that it exists
• getting $_POST["foobar"] without checking that request method is POST
• not setting default case in switch case for header

[simonLeary42]

The main problem with setting the default switch case to "error invalid data ..." is that all forms need to ignore and pass along form_type=clearView so that the "become another user" / "revert to self" functionality will work from header.php. I think a solution is to move that functionality into a function like UnitySite::handleBecome and let each form execute that function. With this solution, all forms would also have to redirect to clear the post data (#233).