Refactor routing, changes API URLs (#1171)

* Make authentication part of the server, so it can be reused outside the Native API

This commit has broken tests after a rebase

* Serve frontend assets from `server`, and not from Native API

* Change Native API URL

* Fix auth tests

* Refactor server authentication

* Simplify authProvider, now subsonic token+salt comes from the server

* Don't send JWT token to UI when authenticated via Request Header

* Enable ReverseProxyWhitelist to be read from environment

server/app/playlists.go

@@ -16,6 +16,8 @@ import (
 	"github.com/navidrome/navidrome/utils"
 )
 
+type restHandler = func(rest.RepositoryConstructor, ...rest.Logger) http.HandlerFunc
+
 func getPlaylist(ds model.DataStore) http.HandlerFunc {
 	// Add a middleware to capture the playlistId
 	wrapper := func(handler restHandler) http.HandlerFunc {