show count of unread messages in Inbox

a count is shown of unread messages. The last access time is stored in localStorage,
so the count is persistent. when ever you reload the inbox it's reset to zero (which is good enough for now...)
This already exists in undercase, see https://github.com/pfraze/phoenix/blob/20c693c572f1c54f0cc382e1e14ae89c30ef8482/web_frontend/src/home.js#L18-L19 and https://github.com/pfraze/phoenix/blob/20c693c572f1c54f0cc382e1e14ae89c30ef8482/web_frontend/src/lib/ws-rpc.js#L22-L23