Users cannot create an Ignore filter using Watch toggles when they are all OFF

Hi! The ticket was created following the exploratory tests performed on XWiki 15.5. RC1 and was noticed because yes, in the past the user could create those ignore filters by switching the watch toggles. I personally don't know how much this is used by other users in production (or if it's used at all) now that the default behavior is to enable the watch filters only for desired pages.

Moreover, if we decide to change the behavior of the ignore filter to not take into account anymore mention events, then the ignore mentions UC described on the ticket has no scope anymore.