Servers from config

*Not complete, don't merge yet*

This is a start on the work to have Servers configured in config files rather than in the database. I have added Server config to config.groovy in a way that I think will work but feedback on this approach is welcomed. I have updated HostVerifier so that proxy requests use the new Server configuration to validate proxy URLs. There are other usages of Servers which haven't been changed over, yet.
I wouldn't go with `allowedServers` because validating proxy requests is just one of the reasons we have this list of servers. It could be just `servers` or maybe `knownServers`?