Conversation message stats

[jerith]

We will soon have the tools (see praekeltfoundation/vumi#893) to extract various stats (specifically the unique address counts that we lost recently, but others as well) from the message store for arbitrary time ranges. It would be handy to have a way to get at these in the conversation UI.

My first thought is a conversation action that asks for a time range and then sends an email (because it's async and doesn't matter if it takes a while to walk the indexes and such) but there are probably better ways to do it.