HTTP Server: Serve Grafana with a custom URL path prefix (#17048)

Adds a new [server] setting `serve_from_sub_path`. By enabling 
this setting and using a subpath in `root_url` setting, e.g.
`root_url = http://localhost:3000/grafana`, Grafana will be accessible 
on `http://localhost:3000/grafana`. By default it is set to `false` 
for compatibility reasons.

Closes #16623

pkg/api/http_server.go

@@ -30,7 +30,7 @@ import (
 	"github.com/grafana/grafana/pkg/setting"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
-	"gopkg.in/macaron.v1"
+	macaron "gopkg.in/macaron.v1"
 )
 
 func init() {
@@ -227,6 +227,10 @@ func (hs *HTTPServer) addMiddlewaresAndStaticRoutes() {
 
 	m.Use(middleware.AddDefaultResponseHeaders())
 
+	if setting.ServeFromSubPath && setting.AppSubUrl != "" {
+		m.SetURLPrefix(setting.AppSubUrl)
+	}
+
 	m.Use(macaron.Renderer(macaron.RenderOptions{
 		Directory:  path.Join(setting.StaticRootPath, "views"),
 		IndentJSON: macaron.Env != macaron.PROD,