Compose post improvements

[edwh]

• Once the old client goes and we only have one set of code to worry about, we should simplify submission into a single server call rather than the current long process in store/compose.
• ComposeGroup - In the old version we had some explicit code to spot when a group we find near to a postcode has been founded more recently than we saved our preferred group. This is so that when a new group has started, we will direct existing active members over to it rather than remember their last group and keep posting to it. We should do something similar.