Messages sent to a group of a subwiki will not be seen by global users who are member of the group

Was: XWIKI-10279.

Steps to reproduce:

1. Login with Admin
2. Create a new global user named "globalUser"
3. Create a new subwiki: select "Only global users are available in the wiki" as the User Scope and add globalUser as a member
4. You'll get a merge conflict for XWiki.XWikiAllGroup: keep the current version of the document and click Continue
5. Edit Main.WebHome inline and add the Message Sender gadget
6. Create a new group in the subwiki, add globalUser as a member
7. Send a message to that group

-> globalUser won't receive the message.