Disable add-on updates in Icedove

In config/chroot_local-includes/etc/icedove/pref/icedove.js we have pref("extensions.update.enabled", true);. We should disable it!

Feature Branch: kytv/bugfix/10428-disable-addon-updates-in-icedove

Original created by @anonym on 10428 (Redmine)

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