Mention an user from a subwiki

Preconditions: 

Email sending must be configured on the subwiki.

NOTE: Starting XWiki 11.8RC1 the main wiki's mail configuration is used when there's no overridden configuration at the subwiki's level, see XWIKI-12435

1. Login as Admin
2. Create a subwiki
3. Create a local user on it (e.g. UL1) with a valid email address
4. Login as UL1
5. Go to Notifications Preferences > Applications section and make sure Mentions toggles are ON for both Notification Menu and Email (and all Pages toggles are set to OFF)
6. Login as Admin
7. On subwiki, edit a page with CKEditor (in-place or WYSIWYG)
8. Type '@' to display users mention list
9. Select the local user from that subwiki (e.g. UL1)
10. Click 'Save & View'
11. Trigger the 'Notifications daily email' job from Scheduler.WebHome (on subwiki)
12. Login with the local user UL1
13. Check the notifications list and email