Use fully relative URLs to allow reverse proxying from arbitrary URLs

This patch set makes all URLs pointing inside birdreader fully relative, not starting with `/`.

This allows birdreader to be reverse proxied from any URL, as long as the proxy takes care to remove the additional prefix. For example the external URL `http://example.com/prefix/for/birdreader/` could be proxied to the internal server `http://localhost:3000/`.

(Additionally it makes one `http` link into `https` to pass more security tests when birdreader itself is accessed via `https`)