Allow other (specified) users to edit your superhub pages

The superhub in the DACH region is getting help from third party developers to improve the HTML code of their superhub pages. They require a way to allow other users to edit the pages without the need to assign the complete superhub role to these new users.

This task is about adding a new field "Additional editors" to the superhub pages that allows other users different from the superhub manager to edit these pages in order to help decentralized the maintenance of the code in these pages.