[Web] Add users to custom groups

[exalate-issue-sync]

Description

User Stories

• As a user I want to add users to custom groups so that I can use groups for my individual purposes.

Value

Acceptance Criteria

• Groups created in Administration Settings are "custom groups"
• Groups provisioned via the IDM are read-only groups "ro-groups" (members assignments and groupname can not be changed)
• Custom groups can be created by users with the proper permission
• users can get added to custom groups by users with the proper permission
• ro-groups are visually distinguished via a 🔒 Lock icon
• In group management custom groups are not editable. Don't show quickaction and context menu entry for those.

Like as is at the moment, with the difference that

• groups a user can be added/removed are custom groups only

Note: For koko we have to deal with 2 kinds of groups

• read-only groups: provisioned by the mebis IDM. The Instance-School-Admin can not edit these groups.
• custom groups: groups that can be managed by the Instance-School-Admin

Technical note: vue-select textarea allows to remove group items with the keyboard (backspace/delete keys). If it's "too complicated" to suppress this, we'll accept that flaw and rely on the backend error when saving. The most important part is that the UI doesn't suggest that removing groups is possible.

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

[exalate-issue-sync]

Jannik Stehle commented: Implementation for read-only-support in the OcSelect component has been added via #8727. Now waiting for the server to finish this task.