Support for different configurations

[dnavre]

Notifier should support different configurations when sending out emails. There are cases when one application has to use different SMTP credentials when sending out different types of emails or even different SMTP servers.

It should be possible to specify a default configuration for a given notification type and to override it with another configuration when sending out a specific email by providing the overriding configuration ID.

Information about used configurations should be persisted into the database.