#17 [ENH] Retian fingerprints of spam messages

When a message is confirmed to be spam, compute a fingerprint and persist it for that guild. If the same fingerprint is seen again, alert mods. This would require changes to most existing cogs to centralize "this is a spam message" detection. Actual fingerprinting would be a new cog type. Fingerprint can be calculated off: - message text - image sizes - image names Think about how tolerant to make it. Perhaps calculate a few different fingerprints, e.g. with and without image file names. Or compute image fingerprints separately (filename + size).