Replace custom chat with existing solution

[Torgen]

From the wrap-up thread. Consider something to replace our homegrown solution. Strict requirements:

• Embed in the blackboard
• Its own mobile app (Android and iOS)
• Works for team members who don't want to create an account/link one with their identity
• API for blackboard to e.g. create channel with puzzle is created
• Hubot adaptor or API supports realtime so we can write our own
• Preserves history

There are other nice-to-haves, like pinning messages (which would close #281), but I think these are the drop-deads. If we can't find something that satisfies these, close as infeasible.

Options to consider:

• Slack
• Flock
• Rocket.chat
• Mattermost
• Discord (some other teams said in https://www.reddit.com/r/mysteryhunt/comments/7s3jhe/how_does_your_team_coordinate_and_keep_track_of/ that they use it)

(More can be added.)