Add and remove items from omeroweb settings

Add elements to an omeroweb list or dict:

    bin/omero config set omero.web.apps.extend '["test"]'
    bin/omero config set omero.web.ui.menu.dropdown.extend '{"TEST": "Test"}'

Remove elements from an omeroweb list or dict:

    bin/omero config set omero.web.ui.right_plugins.remove '[["Preview", "webclient/data/includes/right_plugin.preview.js.html", "preview_tab"]]'
    bin/omero config set omero.web.ui.menu.dropdown.remove '["LEADERS"]'

Notes:

* Use setting name and append ".extend" or ".remove"
* .extend settings must be strings representing JSON lists to extend lists, dicts to extend dicts
* .remove settings must be strings representing JSON lists with items/keys to be removed from lists/dicts
* .extend and .remove settings can be combined with both default or custom settings

See included tests for more examples.