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

cmd/root.go

@@ -74,7 +74,7 @@ func startServer() (func() error, func(err error)) {
 	return func() error {
 			a := CreateServer(conf.Server.MusicFolder)
 			a.MountRouter("Subsonic API", consts.URLPathSubsonicAPI, CreateSubsonicAPIRouter())
-			a.MountRouter("WebUI", consts.URLPathUI, CreateAppRouter())
+			a.MountRouter("Native API", consts.URLPathNativeAPI, CreateAppRouter())
 			return a.Run(fmt.Sprintf("%s:%d", conf.Server.Address, conf.Server.Port))
 		}, func(err error) {
 			if err != nil {