Allow server mods to have an officially supported /warn functionality

[athomas94]

Right now, there is an official warn function that the automod in a server can utilize, but it seems crazy to me that there's not a manual way to trigger this warning, unless I'm crazy. I don't want to use a bot because I'm an admin and not the server owner, so permissions can get weird with those, and I would love to have the official discord theme on the messages