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

We need a variable to save the pre-Test Day topic, to restore it after the Test Day.

I have been thinking that the bot runs only in #qa (fennecbot appears to run only in #mobile, though it advertises to several other channels). Is that not correct?