Local BD for conversation store (Slack optional)

[malanjary-tsri]

Currently Slack is the dependent store of conversations and user interactions. There already exist a local DB store for metadata, --dry-run, and private thread conversations (start as local DB).

This should be augmented to:

1. Expand the conversation database in Postgres to include content beyond just metadata and other Slack features (channels, private-channel, "@" bot)
2. Find all Slack interfaces in the code and re-wire them to read/write to the local database instead
3. Expand the syncing between Slack and the local database so that user (non-bot) interactions can still exists via the local database

Furthermore, this separation should allow for disabling Slack entirely.