Allow the notification macro to disable watching the whole wiki

Before XWIKI-19070, the default behavior for the notifications was to implicitly watch the whole wiki by default.

Afterward, it was not the case anymore. Which is logical but was preventing the notifications macro to show the full set of events of the wiki (e.g., on the dashboard or on the user profile).

We decided to allow the old behavior back in the notification macro, but there is currently no easy way to disable watching the whole wiki.

A new parameter must be introduce, or possibly a special value on the wikis parameter (e.g., "-").