Add a missing release notes for CVE-2013-4795, fixed in 1.6.18.

This was a XSS vulnerability that was fixed as part of Djblets.
Merge branch 'release-1.6.x' into release-1.7.x
Add a missing release notes for CVE-2013-4795, fixed in 1.7.12.

This was a XSS vulnerability that was fixed as part of Djblets.