Use container data stored in database to populate tree view

I've made the changes I think are needed for the API to work better with the ExtJS code. It would be good to use `research_group` instead of `owner` and include the comment somewhere. The colour is, as you probably guessed, in hex. I'm flexible on that - I just figured it would be simple enough to use.

The sample capacity is incomplete as we don't actually store samples at the moment!

I'll see about adding some fixtures to make working with the container data a bit easier.

Fixes #6