Fix diff comments loaded by AJAX to load comment with diff in discussion tab
Closes #3529 (closed).
Demonstration
This commit also fixes two minor issues:
- Ensure notes that the current user is not allowed to see are not returned in the AJAX notes loading
- Ensure the notes counter badge is decremented of 1 instead of 2 (fig. 1 demonstrates the issue, fig. 2 shows the fix)