Improve Slack integration

[janaka]

• 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
• tech debt: switch to llama index pipelines to better control RAG compositions.