Load the server announcements from a file instead of hard-coding them into the app

As of #26 (closed) we have a hard-coded grid configuration that is wrong everywhere except on my home LAN.

We may end up shipping a hard-coded grid configuration for a while but it would be nice if it were a bit easier to tweak that configuration than it currently is. Right now you have to edit Haskell source and rebuild.

We can load a servers.yaml file. Perhaps it would be a good idea to ship the hard-coded configuration in such a file and load it from the app.