Introduce alerts for user-configured events i.e. friend has joined a server

[TwitchPlaysBF2]

User should be able to set up alerts for desired scenarios.

The mechanics may be generic and apply to many scenarios.

A good start would be to notify the user about a friend joining a server.

Keep configuration simple for now:

• Pressing a bell icon for a friend within the friend list, enables alerts when the friend joins a server