[Web] User Group assignments: show only selectable groups

[exalate-issue-sync]

Description

User Stories

• As an Admin who wants to add users to groups, I want that only not-yet added groups are suggested in the list, so that I can focus on finding the right group.

Value

Acceptance Criteria

• Show only groups in the list, that are not already selected
• Bonus: sort list alphabetically

Definition of ready

[ ] everybody needs to understand the value written in the user story
[ ] acceptance criteria has to be defined
[ ] all dependencies of the user story need to be identified
[ ] feature should be seen from an end user perspective
[ ] user story has to be estimated
[ ] story points need to be less then 20

Definition of done

• Functional requirements
  [ ] functionality described in the user story works
  [ ] acceptance criteria are fulfilled
• Quality
  [ ] code review happened
  [ ] CI is green
  [ ] critical code received unit tests by the developer
  [ ] automated tests passed (if automated tests are not available, this test needs to be created and passed
• Non-functional requirements
  [ ] no sonar cloud issues