GN-4953: fix issue with meeting absentees not shown

[elpoelma]

Overview

This PR fixes an issue in the participation-list component which displays the attendance of the different mandatees in a meeting. It corrects the approach in how absentees are fetched/retrieved.

connected issues and PRs:

GN-4953

How to test/reproduce

• Open the app
• Open a meeting
• Modify the attendance of a meeting (and add some absentees)
• The absentees should show up correctly

Challenges/uncertainties

When relationships of a model are correctly loaded in the ember-data store, you can not retrieve these using javascript '.' getters. You can either use the .get('...') or .belongsTo('...') methods.

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations