Allow different messages per channel

Re-ticketed from #3

Allow a custom message for different rooms that might be entered. We don't have a need for this yet (that I know of), but I can imagine it being helpful down the road.

Start by tracing the repository’s Slack greeting flow and how the current message is selected for a channel. Define how room-specific messages should be configured and what should happen when no custom message exists. Done means different rooms receive their configured messages without breaking the existing default behavior.

Written by the indexing model from the issue text.