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/app_suite_test.go

@@ -9,7 +9,7 @@ import (
 	. "github.com/onsi/gomega"
 )
 
-func TestApp(t *testing.T) {
+func TestNativeApi(t *testing.T) {
 	tests.Init(t, false)
 	log.SetLevel(log.LevelCritical)
 	RegisterFailHandler(Fail)