Support more canteens

It would be cool if we could support more canteens/mensas (other than those on the CAU campus). Since we already integrate [`swift-mensa`](https://github.com/fwcd/swift-mensa), which supports many German university canteens via the OpenMensa API this should be mostly a matter of improving the (currently hardcoded) mapping from names to canteen ids. Some ideas:

- Perhaps either `MensaCommand` (or another command) could be used to add 'aliases' for canteens dynamically? These aliases could be persisted in a file like `local/mensas.json` (or similar).
- Perhaps we could even use the built-in Geocoders to query the OpenMensa API for canteens near that location (and ideally cache the results too)