Add notifications feature

[rafalp]

Misago does plenty of things for notifications feature like subscription levels, following and mentions. We could do in-app notifications for those.

Facebook separates new notifications from unread ones. Digit next to notifications icon is number of new notifications you haven't seen yet. When you open the window, this number is zeroed, but notifications are still there, with unread status. When you click on notification, its marked as read and you are redirected to its target. Look at the console, Facebook fires notifications_update_seen_or_read GraphQL mutation on click.

[rafalp]

I can't rely on firing an event on clicking the alert because Misago is not SPA, but I can have all alerts link to view that will set alert as read and redirect user to alert's target.

Misago would use redirecting view anyway, in order to be able to have a url to the post. Except this one will prepend it's URL resolution logic with extra steps to retrieve alert, mark it as read and then redirect user to alert's target.