Change the `/editmessage` command to just rewrite the get-roles-message

There seems to be very little purpose for a committee member to manually enter the text to replace a message that TeX-Bot has previously sent. Instead every command that can create a new message by TeX-Bot should have an `edit` boolean flag, along with the ID of an existing message, to rewrite the text defined originally for that message.

There will be no need to replicate the message text once #90 is complete.