Forward requests to Lemmy backend

[Nutomic]

https://github.com/LemmyNet/lemmyBB/blob/main/docker/nginx.conf#L67

Then we can get rid of this nginx block. This should also make it easier to use lemmybb with other reverse proxies.

API endpoints can be forwarded too, conditionally with an env var LEMMYBB_ENABLE_API.