Handle index.html correctly in custom served folders (#3168)

* getStringMapString return nil if key not found
* Refactor custom routes. Handle /index.html

internal/api/server.go

@@ -231,18 +231,9 @@ func Start() error {
 	// Serve static folders
 	customServedFolders := c.GetCustomServedFolders()
 	if customServedFolders != nil {
-		r.HandleFunc("/custom/*", func(w http.ResponseWriter, r *http.Request) {
-			r.URL.Path = strings.Replace(r.URL.Path, "/custom", "", 1)
-
-			// map the path to the applicable filesystem location
-			var dir string
-			r.URL.Path, dir = customServedFolders.GetFilesystemLocation(r.URL.Path)
-			if dir != "" {
-				http.FileServer(http.Dir(dir)).ServeHTTP(w, r)
-			} else {
-				http.NotFound(w, r)
-			}
-		})
+		r.Mount("/custom", customRoutes{
+			servedFolders: customServedFolders,
+		}.Routes())
 	}
 
 	customUILocation := c.GetCustomUILocation()