move database to REST API

[pmeier]

Closes #65. This is a major refactor. Here are the highlights:

• The Python API no longer depends on the database. This means the modules ragna.core._state and ragna.core._orm were moved (and refactored) into ragna._cli.
• Due to the point above, I was able to simplify the "helper" objects in the Python API. Most of them no longer carry an ID, since that is no longer necessary there and is now handled by the REST API. The two exceptions are ragna.core.Document and ragna.core.Source. ragna.core.Sources are generated by the selected source storage and thus we cannot make any assumptions on that other them being unique for a given source. Still, we want to link these sources to documents in our database for the REST API.
• ragna.core.SourceStorage.retrieve now also takes the documents to make it easier to link the ragna.core.Sources to them. Although not relevant right now, this also enables the use case of having a central source storage that doesn't actually stores documents on user request, but only retrieves sources from pre-stored documents.
• ragna.core.RagnaId was removed in favor of plain uuid.UUID
• Closes deleting vs. closing chats #62, since it would be too tedious to do the refactor for the /close endpoint when we are going to delete it afterwards anyway. I've renamed the /start endpoint to /prepare. You can now send a DELETE request to /chats/{id} to delete a chat.

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[pmeier]

Merging this without review since I have another branch in backlog that depends on this one.