Add support of utf8mb4 for mysql (#6992)

modules/auth/user_form.go

@@ -23,6 +23,7 @@ type InstallForm struct {
 	DbPasswd string
 	DbName   string
 	SSLMode  string
+	Charset  string `binding:"Required;In(utf8,utf8mb4)"`
 	DbPath   string
 
 	AppName      string `binding:"Required" locale:"install.app_name"`