dev_5_0: Add omero config append/remove subcommands with default value handling for omero.web.* variables

This PR addresses https://trac.openmicroscopy.org.uk/ome/ticket/11201

Changes included are:

* addition of bin/omero config append and bin/omero config remove so that configuration using lists of strings can easily be manipulated, e.g.

        bin/omero config append omero.web.apps '"webfigure"'
        bin/omero config append omero.web.apps '"webtagging"'
        …
        bin/omero config remove omero.web.apps '"webfigure"'

* addition of corresponding unit tests under test_prefs.py
* 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"]]

--rebased-from #2030