Global users are not suggested when trying to send them a message from a subwiki

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. Select User from the drop-down list and begin typing "globalUser"

The search suggest will display "User not found".

I have tested the same scenarios on the main wiki and a subwiki with local users and I couldn't reproduce this issue.

Re XWIKI-6668: Also if you create a new group in the subwiki, add globalUser as a member and then send a message to that group, globalUser won't receive the message.