Deprecate and remove Message Stream from XWiki Standard

[surli]

@lucaa

However, the last time I checked, the wiki membership feature was using the message stream to send notifications to users (to admin when a user wants to join, to users when they’re invited by admin), so we may need to refactor that. I think there was an issue about improving those notifications, but I can’t find it now (maybe it was already done?)

So I checked this and indeed right now there’s a dependency to Message Stream. But, the feature is broken: apparently message stream related notifications are not received anymore since Loading.... Meaning that indeed users can write message to admin, but those won’t be received, they will only be displayed in the administration.
We will need to check what exactly we want to do with this when dropping Message Stream. Cleanest solution IMO would be probably to introduce a dedicated TargetableEvent for the Invitation application, and use those.