issue 31: update channel topic (#31)

hi @whimboo !

with apologies for the untidy commit list, suspecting this requires re-forking the repo to fix, and not sure what that would do to existing pull requests

This listener receives a topic message whenever the bot joins a channel as well as when the channel topic changes.

Here we're saving the pre-Test Day topic so that we can restore it when the next Test Day ends.

Again, I have been thinking that the bot runs only in qa.