Grouping: Store groups in local store

[relative-progressio]

In order to show the groups, the groups should be read from the local store. If a group is stored in the local store, the group should be displayed. If the group is not in the local store, a service should look up if a server-side-group is available and display the group.

After all the groups are always at the local store.

[relative-progressio]

Implementation based on #62