Add Notifications and Friends

Update site's header:
- add number of unread notifications.
- and number of new friendship requests.

New models: Relationship and Notification.
New controllers: FriendsController and NotificationsController.
Add two migrations that creates tables.

Update cabinet:
- add friends link. It leads to your friends/followers/subscriptions/requests.
- add notifications link. It leads to list of notifications.

Update User model:
- add method addNotification().

Update the user's page:
- add current relationship status.
- add links to change relationship (add in friends or otherwise delete from friends).
- add links to user's friends/followers/subscriptions.

Update theme/style.css:
- add <info_alert> class.