Add proxy_base_prefix option to help run in a subdir behind a proxy

I have a setup where Radicale is running as the normal process, and
nginx is acting as proxy for Radicale for requests under /radicale/.
This means that all requests to nginx that go to /radicale/ are sent to
a radicale server that expects requests coming at /.

Obviously, all the href need to have the /radicale/ prefix to have
things work. So a new option proxy_base_prefix is added to allow having
such a setup.

All href that are sent in the replies will have the proxy_base_prefix
prepended.