Badge with number of unread messages

Somehow determine if a message has been read. We can maybe do this by seeing if it got mounted.

Then just display a badge with the number of unread messages.

We could store the number of unread messages in redux, increment it when a message is received and then decrement it when the message gets rendered.

The issue does not name files or tests. Start by locating the message-rendering component and the Redux message-handling entry point; determine how received messages become read, then verify that the badge count updates when messages arrive and render.

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