[12580] Improve chat channel code:

- Remove unnecessary calls to GetPlayer
- Invert "if" statements to reduce nesting
- Cleanup code

Signed-off-by: stfx <stfx@hotmail.de>
[12581] Implement 2 chat channel responses

- CHAT_PLAYER_NOT_BANNED_NOTICE
- CHAT_PLAYER_INVITE_BANNED_NOTICE

Signed-off-by: stfx <stfx@hotmail.de>