Access level to control the right to change rights

We should add a new access level, "rights", which controls who can edit the access rights.

Currently, this is controlled by the "edit" right, but it means that a user who can just view and edit a page can give himself other rights, like delete or comment (fortunately it is not possible to grant higher rights on a page).

12.x update of the situation:

Currently on the nested spaces / pages hierarchy, the situation is as follows:

• at a terminal page level (page that cannot have children), whoever has edit rights can update the rights of the page. This is presented on the UI in the edit page menu, for advanced users
• at a non-terminal page, there are 2 types of rights that can be set, and both of them can be set and can be different:
  • on the page itself
    • these require edit right on the page, and are presented on the UI both in the edit page menu, for advanced users and in the administer page menu, for users having admin right on that hierarchy (regardless of whether they are advanced or not)
  • on the page and children (what used to be rights on the "space")
    • these require admin right on the hierarchy (on the "space") and are presented in the UI in the administer page menu for all users that have the right (regardless of whether they are advanced or not)