Default source storage and assistant in the UI

[pmeier]

We currently set the default for the source storage and assistant like this

https://github.com/Quansight/ragna/blob/62098457588cef258f4ea6a8b35989257559a27f/ragna/_ui/modal_configuration.py#L171-L180

where components is just the return value of

https://github.com/Quansight/ragna/blob/62098457588cef258f4ea6a8b35989257559a27f/ragna/_api/core.py#L77-L87

Instead of just setting the first value we get back as default, we should impose a better order. I propose the following:

source storages

1. Use Chroma if available.
2. Use any non-demo source storage if available
3. Use the demo source storage

assistants

1. Use OpenAI gpt-3.5-turbo-16k if available
2. Use any non-demo assistant if available
3. Use the demo assistant

That way, users will have a good experience out of the box if they have the proper requirements set.

cc @nenb

[petrpan26]

I can take a stab at this, it look straightforward enough. Do you have any technical thoughts on this