Use %DISPLAYNAME% instead of player prefix and suffix in multichat -> IRC messages

[CSmither]

Currently player prefix and suffix of multichat messages are not parsed correctly for colours. This change brings it more inline with how game chats are handled and uses %DISPLAYNAME% to display the prefixes and suffixes instead of directly accessing them.

• Minor change
• Only affects Multichat -> IRC messages
• Tested on test server with latest Multichat