Incorporate buddy rides into system

[patcon]

Depends on #5

If we're going to start expanding invites, would be nice to capture to chain of trust in the data store.

There are complex ways this could be done, by probably the easiest way is for:

1. existing a prospective member go for buddy ride (untracked),
2. existing members invites new user to chat, and they accept,
3. existing user acks new user with some command, e.g., "@newuser 🚲🤝🚲"

The bot now can track that @existinguser has vouched for @newuser, and keep track of that for later.