Add opt-out of bot function

[LeoSaucedo]

To make a message contents privileged intent to Rikka, we must implement a way for a user to opt out of having their message information read. This should be done via DMing the bot, and we should keep a database of User IDs that have opted out of using Rikka within a server. This requires us having a new database column or table containing this information as well as a function implementation for receiving DMs. This action should be reversible and should be able to be reversed via DMs as well.