Deploy to fly, other updates

[sethetter]

• Set up fly.toml config for deploying to fly.io
• Update the Dockerfile to expose port 3000
• Add a .dockerignore to keep certain files out of the image
• Add a requireMention flag to command handlers
• Some type reworking to match what is passed into the app.message handler middleware
• Ensure the slack client is being started properly, with command handlers registered
• Rename KeeperSlackMiddleware to just MessageEvent

Issue