Load default when appending to/removing from an unset omero.web property

This PR extends #1905 by loading the default value from omeroweb/settings.py when a value is appended to or removed from a previously unset omero.web.* property.

    $ omero config get omero.web.server_list
    $ omero config append omero.web.server_list '["test",123,"omtest"]'
    $ omero config get omero.web.server_list
    [["localhost", 4064, "omero"], ["test", 123, "omtest"]]