Fix Thunderbird System policies

Policies can be set system-wide for Thunderbird via /etc/thunderbird/policies/policies.json. Users can also use this directory to set different default preferences. We're currently allowing private-etc similarly for Firefox & derivatives, so I don't see why we shouldn't do the same for Thunderbird.

etc/profile-m-z/thunderbird.profile

@@ -57,6 +57,9 @@ whitelist /usr/share/gnupg
 whitelist /usr/share/gnupg2
 whitelist /usr/share/thunderbird
 
+# Required for system policies
+private-etc thunderbird
+
 # machine-id breaks audio in browsers; enable or put it in your thunderbird.local when sound is not required
 #machine-id
 novideo