Issue Summary

Users receive a misleading notification when mentioning someone on a page, even though the mentioned user has access via group permissions.

Notification pop up message - 

"Share with USER? They don't have access to this content, so we can't let them know you mentioned them. ""

Steps to Reproduce

1. Add a user 'A' to a group 'ABC'
2. Add the user 'A' and group 'ABC' to a space via space permissions
3. Create a new page in that space -> restrict the page -> add the group 'ABC' where user 'A' is a part of it and save it.
4. 
5. Now edit the page and mention the user 'A' (which has access to the page being part of group 'ABC') in the page -> you will see the notification pop up in the left side down corner.
6. 

Expected Results

We should not get the notification pop up as it is not true and the user has access to the page . The user is also able to access the page.

Actual Results

User gets a notification pop up - "Share with USER? They don't have access to this content, so we can't let them know you mentioned them. "

Workaround

Yes, add the user 'A' directly to the restricted page