Allow Gogs to run from a suburl behind a reverse proxy. e.g. http://mydomain.com/gogs/

Conflicts:
	modules/setting/setting.go

Conflicts:
	templates/repo/release/list.tmpl
	templates/user/dashboard/dashboard.tmpl

Conflicts:
	routers/repo/setting.go

modules/setting/setting.go

@@ -6,6 +6,7 @@ package setting
 
 import (
 	"fmt"
+	"net/url"
 	"os"
 	"os/exec"
 	"path"
@@ -31,9 +32,10 @@ const (
 
 var (
 	// App settings.
-	AppVer  string
-	AppName string
-	AppUrl  string
+	AppVer        string
+	AppName       string
+	AppUrl        string
+	AppRootSubUrl string
 
 	// Server settings.
 	Protocol           Scheme
@@ -165,6 +167,12 @@ func NewConfigContext() {
 		AppUrl += "/"
 	}
 
+	url, err := url.Parse(AppUrl)
+	if err != nil {
+		log.Fatal(4, "Invalid ROOT_URL %s: %s", AppUrl, err)
+	}
+	AppRootSubUrl = strings.TrimSuffix(url.Path, "/")
+
 	Protocol = HTTP
 	if Cfg.MustValue("server", "PROTOCOL") == "https" {
 		Protocol = HTTPS