gnu: icecat: Update to 60.8.0-guix1 [security fixes].

Includes fixes for CVE-2019-9811, CVE-2019-11709, CVE-2019-11711,
CVE-2019-11712, CVE-2019-11713, CVE-2019-11715, CVE-2019-11717,
CVE-2019-11719, CVE-2019-11729, and CVE-2019-11730.

* gnu/packages/gnuzilla.scm (%icecat-version): Update to 60.8.0-guix1.
(icecat-source)[upstream-firefox-source]: Update hash.
(icecat): Refresh some stale comments.