New Preference to specify a custom Imagr config file

Right now /Deploy/Applications/com.grahamgilbert.Imagr.plist refers to the config on disk.

I propose having a preference to have the serverurl setting be set to an admin provided URL if desired. MDS would not write out the workflow to this file as it may not be at a writable location.

MDS can still write out the imagr workflow to /Deploy/Config/config.plist for the admin to reference to add to their custom imagr config file.