Fix diff comments loaded by AJAX to load comment with diff in discussion tab

Closes #3529 (closed).

Demonstration

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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)

fig.1 (the bug)

fig. 2 (the fix)