Only show permissions not already assigned when assigning new permissions

[Anderen2]

Is your feature request related to a problem? Please describe.
When assigning new permissions to a role, it's not always easy to get a good overview on which permissions that might be lacking. Especially for large roles with lots of permissions added.

Describe the solution you'd like
It would make the interface easier to work with if the "Select permissions to assign" dialog only shows permissions not already assigned.

Describe alternatives you've considered
We increased the results-per-page to 100 to alleviate the problem somewhat.

Additional context
N/A