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/setting/setting.go

@@ -47,10 +47,11 @@ var (
 
 var (
 	// App settings.
-	Env          = DEV
-	AppUrl       string
-	AppSubUrl    string
-	InstanceName string
+	Env              = DEV
+	AppUrl           string
+	AppSubUrl        string
+	ServeFromSubPath bool
+	InstanceName     string
 
 	// build
 	BuildVersion    string
@@ -205,8 +206,9 @@ type Cfg struct {
 	Logger log.Logger
 
 	// HTTP Server Settings
-	AppUrl    string
-	AppSubUrl string
+	AppUrl           string
+	AppSubUrl        string
+	ServeFromSubPath bool
 
 	// Paths
 	ProvisioningPath string
@@ -610,8 +612,11 @@ func (cfg *Cfg) Load(args *CommandLineArgs) error {
 	if err != nil {
 		return err
 	}
+	ServeFromSubPath = server.Key("serve_from_sub_path").MustBool(false)
+
 	cfg.AppUrl = AppUrl
 	cfg.AppSubUrl = AppSubUrl
+	cfg.ServeFromSubPath = ServeFromSubPath
 
 	Protocol = HTTP
 	protocolStr, err := valueAsString(server, "protocol", "http")