ROOT_URL setting use the default as shown in conf/app.ini (#1823)

The well commented conf/app.ini file that comes with the code shows the
ROOT_URL (i.e. setting.AppURL) as:

    ROOT_URL = %(PROTOCOL)s://%(DOMAIN)s:%(HTTP_PORT)s/

However the installed custom/conf/app.ini file does not include this setting as
shown, and the default in the setting module was hard coded to
http://localhost:3000/ instead of what is shown above.

With this change the ROOT_URL will default to what is shown above if it is not
set in the custom/conf/app.ini.

Of course it is still possible to override the default by adding the ROOT_URL
setting to your custom/conf/app.ini file as usual.

Signed-off-by: Mike Fellows <mike.fellows@shaw.ca>

modules/setting/setting.go

@@ -624,18 +624,6 @@ please consider changing to GITEA_CUSTOM`)
 
 	sec := Cfg.Section("server")
 	AppName = Cfg.Section("").Key("APP_NAME").MustString("Gitea: Git with a cup of tea")
-	AppURL = sec.Key("ROOT_URL").MustString("http://localhost:3000/")
-	AppURL = strings.TrimRight(AppURL, "/") + "/"
-
-	// Check if has app suburl.
-	url, err := url.Parse(AppURL)
-	if err != nil {
-		log.Fatal(4, "Invalid ROOT_URL '%s': %s", AppURL, err)
-	}
-	// Suburl should start with '/' and end without '/', such as '/{subpath}'.
-	// This value is empty if site does not have sub-url.
-	AppSubURL = strings.TrimSuffix(url.Path, "/")
-	AppSubURLDepth = strings.Count(AppSubURL, "/")
 
 	Protocol = HTTP
 	if sec.Key("PROTOCOL").String() == "https" {
@@ -656,6 +644,24 @@ please consider changing to GITEA_CUSTOM`)
 	Domain = sec.Key("DOMAIN").MustString("localhost")
 	HTTPAddr = sec.Key("HTTP_ADDR").MustString("0.0.0.0")
 	HTTPPort = sec.Key("HTTP_PORT").MustString("3000")
+
+	defaultAppURL := string(Protocol) + "://" + Domain
+	if (Protocol == HTTP && HTTPPort != "80") || (Protocol == HTTPS && HTTPPort != "443") {
+		defaultAppURL += ":" + HTTPPort
+	}
+	AppURL = sec.Key("ROOT_URL").MustString(defaultAppURL)
+	AppURL = strings.TrimRight(AppURL, "/") + "/"
+
+	// Check if has app suburl.
+	url, err := url.Parse(AppURL)
+	if err != nil {
+		log.Fatal(4, "Invalid ROOT_URL '%s': %s", AppURL, err)
+	}
+	// Suburl should start with '/' and end without '/', such as '/{subpath}'.
+	// This value is empty if site does not have sub-url.
+	AppSubURL = strings.TrimSuffix(url.Path, "/")
+	AppSubURLDepth = strings.Count(AppSubURL, "/")
+
 	LocalURL = sec.Key("LOCAL_ROOT_URL").MustString(string(Protocol) + "://localhost:" + HTTPPort + "/")
 	OfflineMode = sec.Key("OFFLINE_MODE").MustBool()
 	DisableRouterLog = sec.Key("DISABLE_ROUTER_LOG").MustBool()