Prevent re-running the setup command in a server

[GeordieP]

Currently we have protection against registering the same channel more than once, but it would be better to check for the existence of the bot's initial setup message, so a channel isn't "stuck" as registered - ideally, we would be able to re-register if the init message is deleted by a moderator.

registration message:

• [x] Send a registration code in the init command interaction response msg
• [x] Listen for bot messages (from self) containing a registration code, store msg ID in DB when found

init command handler changes:

• [x] When init command is run or re-run, check for a registered channel init message in the DB
• [x] Check if the message can be retrieved in the server
  • [x] if yes: do nothing; the channel is initialized
  • [x] if no: send a new init message, save the ID in the database for next time (handled by the registration message logic above)

[GeordieP]

There's a snag on this - the API doesn't return a message object for an interaction response.

[GeordieP]

Snag resolved:

• Init command handler creates an incomplete database item, grabs its ID
• Sends an interaction response with no embeds & no components, but content containing a "registration code" using the database ID e.g. LFGREG:123-456-7890
• A message handler listens for bot messages (self messages) where content matches "LFGREG:" <> reg_id
• Use reg_id to update the DB item, storing the message ID
• Edit the discord message, clearing the registration ID content, and adding in the proper embeds (introduction message) and components ('new game' button & any other controls)

After this process, the database keeps track of channels that have had the registration command run in them, as well as the ID of the bot-sent introduction message. We can then use that persisted message ID to ask the API if that message has been deleted, and do logic to prevent unnecessary re-registering as well as any other operations that may need the msg ID (e.g. retroactively editing the init message for all registered channels, to change the text or list new bot features.)