Detect duplicate messages and act based on configuration

[emmiegit]

Related to #191

Server administration should be able to configure two thresholds X and Y. If a member repeats a message X times or more, additional messages are deleted (and a PM is sent to the user). If they repeat it Y times or more, they are automatically dunced as well. Duplicates past a configurable expiration period are reset.

This should apply per-member across the entire guild.