Change the notifications locations inclusion default: don't consider that the whole wiki is watched when nothing is watched

Currently, in the wiki, when no location is "watched" by the user (when no filter exists), the whole wiki is considered watched.
Then, spamming users with notifications is avoided by disabling the page event by default, in the events settings.
Then, when the user watches a page, the page events are automatically enabled also in order for notifications to actually be sent to the user, otherwise watching a page doesn't make sense.

I propose the default to change as follows:

• when no filter exists, the whole wiki is not considered included by default anymore.
• the page events are enabled by default
• when a user wants to start receiving notifications, they will need to subscribe (watch) a location
  • through the buttons on the top right of the page - which will create an inclusive filter for all events
  • through the UI to add a filter from their profile
• an administrator will also be able to setup a default global filter for all the new users on the wiki.

Functionally speaking, this wouldn't change anything for the standard wiki (without any customization / filter settings). For a wiki with customizations (filter settings), the default doesn't matter anymore anyway since the actually set filters will be used and this issue should not change any behaviour when actual filters are set. Thus, I don't think this issue would introduce a regression.

See this thread for a discussion around these incoherences: https://forum.xwiki.org/t/clarify-the-behaviour-of-notification-watch-switches/7471 , the detailed explanation for the change proposed here and some of the other related issues .

Simplifying this would also allow to remove all sorts of magical activation of events, such as XWIKI-14129, that is reported as a bug by XWIKI-17540 , and simplify the functioning of the watch buttons described by Simon in https://forum.xwiki.org/t/clarify-the-behaviour-of-notification-watch-switches/7471 .