Improve Slack integration

[janaka]

• [x] enable setting channel space to nothing.
• [ ] bug: handle when channel doesn't have space associated
• [ ] add assistant per to channel
• [ ] longterm memory - extract knowledge from threads and persist as a space so it can be queried
• [ ] make conversation history saving temporary
• [ ] globally shared knowledge spaces on public data like product tech docs e.g. Terraform
• [ ] Code/GH repo as datasource
• [x] tech debt: switch to llama index pipelines to better control RAG compositions.