Add casual "Mode of The Week" (MOTW) matchmaker

It would be cool to have a casual matchmaker that automatically changes configuration on a fixed interval (e.g. every week). At one point we discussed the possibility of having this be its own micro-service that communicates with the lobby server via rabbitmq, but all the details of that haven't been fleshed out yet, and I think it would make sense to start by implementing this in the server.

We would need a way to define a bunch of different game modes, either in the database or through a config file. We probably also need to add descriptions to matchmaker queues, so that the MOTW queue can provide a short description of how what the current game mode is and how to play it.

Some things to consider:
- What rating to use (global, or a new hidden rating)
- What team size to use
- Would the client be able to handle the team size changing dynamically?
- Would we need to make the team size change reflected in the API?
- Where to configure game modes
- How to localize game mode descriptions
- How to pick which game mode to play each week
- It would be nice if we could define special event game modes that only happen during certain weeks, e.g. halloween

This will be a relatively large project.