investigations: Use code block for exploits.txt

[FloatingMilkshake]

Closes #324

Adds the bool useCodeBlock parameter to InvestigationsHelpers.SendInfringingMessageAsync, set to true when auto-warning for words matching exploits.txt. This puts the text going into the embed description in #investigations into a code block.

This will also use a code block for the "Match" field, as that usually has the same issue as the embed description. That part is not controlled by useCodeBlock however, as it looks like there are other cases where extraField (what is used to add the "Match" field) is passed a code block manually

This PR also adds the missing parameters from the SendInfringingMessageAsync overload that accepts a MockDiscordMessage to the one that accepts a DiscordMessage, just for consistency

[Erisa]

Approved with 2 nitpicks