Fix duplicate sub-path for avatars (#31365)

Fix #31361, and add tests

And this PR introduces an undocumented & debug-purpose-only config
option: `USE_SUB_URL_PATH`. It does nothing for end users, it only helps
the development of sub-path related problems.

And also fix #31366

Co-authored-by: @ExplodingDragon

modules/setting/server.go

@@ -40,16 +40,16 @@ const (
 	LandingPageLogin         LandingPage = "/user/login"
 )
 
+// Server settings
 var (
-	// AppName is the Application name, used in the page title.
-	// It maps to ini:"APP_NAME"
-	AppName string
 	// AppURL is the Application ROOT_URL. It always has a '/' suffix
 	// It maps to ini:"ROOT_URL"
 	AppURL string
 	// AppSubURL represents the sub-url mounting point for gitea. It is either "" or starts with '/' and ends without '/', such as '/{subpath}'.
 	// This value is empty if site does not have sub-url.
 	AppSubURL string
+	// UseSubURLPath makes Gitea handle requests with sub-path like "/sub-path/owner/repo/...", to make it easier to debug sub-path related problems without a reverse proxy.
+	UseSubURLPath bool
 	// AppDataPath is the default path for storing data.
 	// It maps to ini:"APP_DATA_PATH" in [server] and defaults to AppWorkPath + "/data"
 	AppDataPath string
@@ -59,8 +59,6 @@ var (
 	// AssetVersion holds a opaque value that is used for cache-busting assets
 	AssetVersion string
 
-	// Server settings
-
 	Protocol                   Scheme
 	UseProxyProtocol           bool // `ini:"USE_PROXY_PROTOCOL"`
 	ProxyProtocolTLSBridging   bool //`ini:"PROXY_PROTOCOL_TLS_BRIDGING"`
@@ -275,9 +273,10 @@ func loadServerFrom(rootCfg ConfigProvider) {
 	// This should be TrimRight to ensure that there is only a single '/' at the end of AppURL.
 	AppURL = strings.TrimRight(appURL.String(), "/") + "/"
 
-	// Suburl should start with '/' and end without '/', such as '/{subpath}'.
+	// AppSubURL should start with '/' and end without '/', such as '/{subpath}'.
 	// This value is empty if site does not have sub-url.
 	AppSubURL = strings.TrimSuffix(appURL.Path, "/")
+	UseSubURLPath = sec.Key("USE_SUB_URL_PATH").MustBool(false)
 	StaticURLPrefix = strings.TrimSuffix(sec.Key("STATIC_URL_PREFIX").MustString(AppSubURL), "/")
 
 	// Check if Domain differs from AppURL domain than update it to AppURL's domain