AutoConfigBackup - user-config-readonly priv still does backup

[phil.davis]

A user with the priv user-config-readonly cannot change the config. This is handled correctly in write_config().
But AutoConfigBackup still makes a backup each time that read-only user presses Save somewhere.
It would be nice if AutoConfigBackup also checked for user-config-readonly priv.

[phil.davis]

Related bug report: https://redmine.pfsense.org/issues/4034