[Dune] More flyspray permission for contributors

[Christoph Grüninger]

Good evening,
Flyspray is cumbersome for people with a restricted permission
level. I propose the make a new group contributors and give
them the right to take the ownership of a task, close tasks
that are not their own's, add votes and edit a task.

I am aware of the fact that we have not yet named 
contributors.
Maybe this would be a reason to make up for this.

Sleep well,
Christoph