Deprecate chat commands that were replaced by slash commands

[cascer1]

To keep everything in a single place, we should deprecate (and eventually remove) any chat commands that were replaced by text commands.

At first, I think we should still execute the command, but also include a notice that the command will soon be removed in favour of the slash command (with an example). Also add a cooldown to the text command to motivate people to use the slash command instead.