Take a step closer to a flux-like architecture for group data, for
the purposes of providing features that require it.
Now the app has a single GroupStore that can be poked to fetch
updates for a particular group.
In order to provide feedback when adding a room to a group, the group summarry store needs to be extended to store the list of rooms in a group. This commit is the first step required.
The next step is to get the GroupRoomList listening to updates from GroupStore and expose the list of rooms from GroupStore.
(We're running out of words to describe the hierachy of things that store things)
