- use MimeMessage with Content-Type text/hmtl
- use a single template for subject and body
- use a Spring aware TemplateResolver

It turns out that imho when talking about application groups (identified
by name) and application instances it's just logical to speak of
instances and applications. So the Application class get's renamed to
Instance. So an Application is now a set of Instances with the same name

Added documentation and a sample on how to secure the SBA-Server and
leverage the new login ui-module

closes #361

This adds a new FilteringNotifier which can use filters to suppres certain
events from being notified of. Two filter implementation are provided, one
filtering by name and one by id. Additional when a FilteringNotifier bean is
present, the autoconfiguration creates a REST-controller to add and remove
filters at runtime.