Add ability to configure globally the default frequency used for email notifications

It is not possible for an administrator to use the tools at Global Administration > Social > Notifications to set the Email Frequency value globally for current users and/or define a default value for this setting which will be assigned to new users.  Other Notification settings can be globally adjusted in this administration page, but Email Frequency seems to be missing.

And modifying the global object's property NotificationEmailPreferences#interval in page XWiki/Notifications/Code/NotificationAdministration directly via the object's editor has no effect.