Enhancement: Move webhook messages in editor

[kouwei32]

The ability to swap the places of messages in the editor would be nice, so that if something needs to be inserted between other ones, you don't have to copy-paste the raw JSON around (which is currently broken for embeds anyways, see #46). Conflicts with the edit by link feature can be remedied by warning the user that the messages won't be in order if the user swaps messages while edit links are active.