Hide Notifications "watch page/space/wiki" buttons when Pages application is disabled in preferences

By default, the Pages Application in Notifications Preferences is disabled (both in Notifications menu and in Emails).
 
As a result, accessing the Notifications dropdown menu, the "watch" buttons are disabled and they show a tooltip saying you need to enable the Pages app in Settings.
 
To be consistent with XWiki's UX, those buttons should only be visible when the Pages app is enabled. The rest of the Notifications dropdown should remain as it is, since other apps could send notifications and it is needed to display them, except maybe when both Notifications and the (old) Watchlist features are disabled from xwiki.cfg/xwiki.preferences (this should could be done as a separate issue or under the same issue).
 
We do not usually display disabled options in XWiki's UI.