Feature/capture chunks

[gecBurton]

Context

As a User I want the django app to persist the specific text chunk referenced in RAG so that I can know exactly which part of the document has been referenced.

Changes proposed in this pull request

1. I have renamed the ChatMessage source_files to old_source_files
2. I have created a new model called TextChunk to capture the relationship between the ChatMessage, File and the specific text used in RAG
3. I have added a new m2m field to ChatMessage called source_files that, like the old field, is a m2m field to Files but has a through relationship via TextChunk
4. I copy over the data in old_source_files to source_files
5. I do not delete old_source_files in case something goes wrong and or we want to revert this change in production

Guidance to review

Relevant links

Things to check

• I have added any new ENV vars in all deployed environments
• I have tested any code added or changed
• I have run integration tests

[jamesrichards4]

LGTM but I don't think I can really review the Django side of things in any detail.

On tying this up with other changes, this is still worth doing as retaining Chunks in parts of core-api is my plan short term. Replacing all the way through the system would be a scary change and I think we can do it incrementally.