(TK-50) Require user to specify default server

This PR adds support for the user specifying a default server. The user will now be required to specify a default server under the `:default-server` key in the webserver section of their trapperkeeper configuration when using a multi-server configuration. Having a server with id `:default` is no longer required.
I don't think you need an atom for this; I think you could use a combination of `filter` and `first`, right?