Fix nuxeo-flavors-unicolor addon

The contributions to flavors from the Unicolor Flavor addon are overriden by the nuxeo-platform-ui-select2 bundle, and the "Nuxeo theme" flavor is presented twice.

The nuxeo-flavors-unicolor addon should be deployed after ui-select2, and nuxeo-platform-ui-select2 should not override the default flavors, as it only needs to define new styles.