slug checking fix
csrf_token upon input errors fix
updated_by_name PHP notice fix upon input errors
create empty updated_by_name if it's not set from $_POST
create hidden input field to store created_by_name
it's used by View when input errors occur
prevent displaying empty `<ul></ul>` for non-existing but $_COOKIE expanded rows
moving updated_by_name unsetter to Page->beforeUpdate
Merge upstream/master into page-edit-fixxx
implementing $page->getColumns()