Improve `MANUAL_MODERATION_WARNING_MESSAGE_LOCATION` validation

Currently we do not do any validation on input for this variable. A regex or other checks should be implemented to ensure that the channel name is valid. This will be tricky as it doesn't seem like discord have clear docs on what is considered valid.

Additionally, it's theoretically possible that a discord channel with the name "DM" exists, which could conflict with the default DM option. A note added to the documentation would likely be useful just to document this.

Locate the handling of MANUAL_MODERATION_WARNING_MESSAGE_LOCATION and the documentation for this setting. First inspect how its current value is read and how channel names are represented, then verify the validation against the issue's Discord-channel constraints. Done means invalid values are rejected and the documentation notes the possible conflict between a channel named "DM" and the default DM option.

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