Implement message notifications

[DimiMikadze]

Let's add message icon near to notifications icon in Header component, that will show count of people that has sent us a message, for example if two users has sent 20 messages, we should show 2 in there, in exactly same styling as in notifications.

Clicking on that icon, should open a dropdown that will contain following list. {User} has sent you a message, after clicking on that item we should be redirected to that specific messages page.

After redirecting to that page that message should be marked as read and should be removed from messages notifications.

This functionality should be real time using GraphQL subscriptions.

PR should be created against implement-private-messages branch, not master.

It is a sub task of: #21