server: block server access via non-local domains

[cebtenzzre]

This PR contains the necessary code to prevent a potential DNS rebind attack of the local server, in which a malicious remote host points their domain name to a local IP address to bypass CORS, allowing a client-side script in a web browser to access the local GPT4All instance. This is exacerbated by the fact that the local server currently provides no authentication mechanism.

TODO: Test this change. done.

[cebtenzzre]

If you add 127.0.0.1 example.com to your /etc/hosts, you can simulate a DNS rebind attack. This PR blocks these requests:

$ curl 'http://localhost:4891/v1/models'
{"data": ...}
$ curl 'http://example.com:4891/v1/models'
{"error":"Access to the server via non-local host example.com is forbidden."}

This also makes it more difficult to expose the local server to the internet, which I believe is a good thing.