Multi-site settings name dangerous and/or confusing

Now that we no longer have the group_name in the settings, the multi-site settings need some refactoring/renaming.

In particular, the 'is_enabled' setting is prone to naming conflicts at the minimum, and at least very confusing for anyone trying to make sense out of this setting if reading it from the database table.

Also the 'event_price_set_domain_id' really should be 'event_price_set_per_domain' as this is not a domain_id stored in this setting but a boolean flag.

Also some of these settings are not accessible in the UI and not documented ...