plt-2610 allow slash command to override username/icon_url (#4378)

api/command.go

@@ -231,6 +231,8 @@ func handleResponse(c *Context, w http.ResponseWriter, response *model.CommandRe
 	if utils.Cfg.ServiceSettings.EnablePostUsernameOverride {
 		if len(cmd.Username) != 0 {
 			post.AddProp("override_username", cmd.Username)
+		} else if len(response.Username) != 0 {
+			post.AddProp("override_username", response.Username)
 		} else {
 			post.AddProp("override_username", model.DEFAULT_WEBHOOK_USERNAME)
 		}
@@ -239,6 +241,8 @@ func handleResponse(c *Context, w http.ResponseWriter, response *model.CommandRe
 	if utils.Cfg.ServiceSettings.EnablePostIconOverride {
 		if len(cmd.IconURL) != 0 {
 			post.AddProp("override_icon_url", cmd.IconURL)
+		} else if len(response.IconURL) != 0 {
+			post.AddProp("override_icon_url", response.IconURL)
 		} else {
 			post.AddProp("override_icon_url", "")
 		}