Slight improvements to FormSpecialPage behavior.

Added a default implementation of doing nothing
to FormSpecialPage::onSuccess to avoid forms that
don't do anything.

Added a default implementation of FormSpecialPage::setParameter
to store the parameter in a protected variable in
case the child class wants to use it.

Made FormSpecialPage::getForm first check whether the
header message text exists before adding it as header
text, for those forms that don't have header text.

Change-Id: Ifb49c3b2f4079a5fbe379c44b63ee2dcee9eeb48
Merge "Slight improvements to FormSpecialPage behavior."