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.

Start with the `/editmessage` command and trace the other commands that create messages. Read issue #90 for the planned message-text handling, then determine how an edit flag and existing message ID should apply consistently; done means `/editmessage` rewrites the original get-roles message without manually re-entering its text.

Scritto dal modello di indicizzazione a partire dal testo della issue.