Remove stories from Product Backlog with existing history

This pull request implements the ability to remove (suspend) a story from the product backlog even it has a history. Before only stories which were not being worked on in a (previous) sprint could not be deleted. 

So now this option is available in the RequirementsBlock for those specific stories. When the user chooses it, the requirement (story) will be suspended, still available as an reference for sprint history, but not listed anymore in the product backlog.

Benefit: keeps the product backlog clean as it should be.