Redesign assign roles page

[timja]

Depends on #765

Don't review the code as it will be quite large till that one is merged

Design input welcome

assign.roles.mov

Testing done

Added users and groups
Tested with 3000 assignments

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[timja]

cc @janfaracik