Dependencies list grouping: Add group-by dropdown to group-level dependencies app

In order to get the group-by feature started, we need to add the dropdown for the grouping option to the dependencies app:

It should only show for the group-level app, but not the project-level. We also need to put it behind a feature flag. The options for grouping by license and project will be added in separate efforts.

Note: We are currently using Vuex, so we need to add the group-by state to the Vuex store.