NotificationAdministration contains a mix of UI and configuration data

NotificationsAdministration contains both the ConfigurableClass to allow configuring globally the notifications options for the wiki, and the actual data for this configuration.
We added previously some default preferences for Mentions, which now led to some conflict when upgrading XWiki.
We now need to:
1. remove the NotificationEmailPreferences from the NotificationAdministration page by default: those should be default values from Java API
2. plan to separate properly UI and data in 2 different files, and how to migrate the actual data on existing wikis (most likely by removing the configurable class, but could also be to move the existing preferences)