Send a notification if DMs were received while the user was offline

[SupDos]

At the moment DMs are very easy to miss if you aren't online at the time. They only show up on web next to the chat button which isn't very obvious, people likely aren't checking the website daily, and neither stable nor lazer notify you when launching the game that you've received new DMs.

I'd like to suggest a new notification that either says something like

You have X unread messages

when launching the game, or just makes a new notification for each message.

[peppy]

Eventually we will synchronise notifications with osu-web, including history, which already has these.