Conversations should get exported and imported

Summary: Currently, Conversations are completely ignored by export/import. This is a problem for backing up conversation-centric Spaces such as SI Playtesting.

In principle, this doesn't look horribly difficult -- there's a bunch of new hooking to do, new XML schema nodes and such, but it doesn't look like rocket science offhand.

The biggest problem is that this is going to be really, really slow at both ends. Export is currently fairly quick, since it is just serializing the Space data structure; this will require looking up and loading the Conversations on every single Thing, which is potentially thousands of roundtrips to Cassandra.

(This highlights an architectural problem: the Space itself knows nothing about the Conversations. That's a deliberate decoupling, and very clean, but means that we have no way to limit the scope of these lookups to just the Things that actually have conversations. This is probably one of the reasons why some operations in the Playtesting Space are so slow.)

Similarly, on the import side this would be ferociously slow, since we need to write a huge amount of data out to Cassandra in many roundtrips. Import is now designed to cope with that sort of slowness, but we would need to enhance the progress bar output to provide better feedback of what's going on.