[Multi-User Part 3]: Separate chat sesssions based on authenticated users (#511)

- Add a data model which allows us to store Conversations with users. This does a minimal lift over the current setup, where the underlying data is stored in a JSON file. This maintains parity with that configuration.
- There does _seem_ to be some regression in chat quality, which is most likely attributable to search results.

This will help us with #275. It should become much easier to maintain multiple Conversations in a given table in the backend now. We will have to do some thinking on the UI.

pytest.ini

@@ -2,3 +2,5 @@
 DJANGO_SETTINGS_MODULE = app.settings
 pythonpath = . src
 testpaths = tests
+markers =
+    chatquality: marks tests as chatquality (deselect with '-m "not chatquality"')