[bisq-network/bisq] Add Notification icon for Mediation and Arbitration chat (#5097)

[huey735]

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### Description

Add a chat icon for notification on new messages to the Mediation and Arbitration chats like the one on the Trader's chat.

![chatNotification](https://user-images.githubusercontent.com/44140017/105354275-3c787680-5be8-11eb-880a-1d043841ab17.png)



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