Fixes #6766 Let Super User view and edit segments created by other users

As a Super User:

 * I can now see all segments that were created for this website by any other user
 * When a segment was created by another user who is not Super User, the segment appears below a new section "Visible to you because you have Super User access:"
 * Such segments are editable by the Super User
 * The only difference when editing someone else's segment, as a Super User, is that "This segment is visible to [ME]" now says "This segment is visible to [SEGMENT_AUTHOR_USERNAME]"
 * One can now search in the  search bar for a username and see all segments created by this user

For all users:

 * New section "Shared with you:" now lists segments created by a Super User, and marked as "Visible to [All Users]"
 * Before segments  shared with me, looked the same as segments I created, which was confusing