Only show permissions not already assigned when assigning new permissions

**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

The issue does not name files, tests, or an entry point. Start by locating the "Select permissions to assign" dialog and its permission-loading or filtering logic, then inspect how a role's assigned permissions are represented. Done means the dialog shows only permissions not already assigned, while still allowing the remaining permissions to be selected.

Written by the indexing model from the issue text.