I'm prepping to make bebot do more things than just be a gitlab webhook
handler, so I've moved the gitlab stuff into its own module (and some of
the matrix helper functions too, for good measure).
The config file also now puts all the gitlab-specific configuration
under a 'gitlab' key.
GitLab really wants you to respond in 10 seconds, and if we're doing a
bit of work (e.g. sending more than one matrix message per event), then
we can easily blow through that.
