Remove ember-drag-drop dependency

[elpoelma]

Overview

This PR includes the following changes:

• Replace ember-drag-drop by ember-sortable
• Some slight adjustments to the styling of the meeting agenda table
• Set the deprecated EXTEND_PROTOTYPES flag to false

How to test/reproduce

• Start the app
• Ensure the meeting agenda table works as you'd expect + the changes are correctly persisted
• There should be no issues caused by disabling the EXTEND_PROTOTYPES flag

Challenges/uncertainties

I made some slight css changes to the meeting agenda table, so it might behave slightly different. This is to make the behaviour consistent with the behaviour on RB, and to better align the styles with how ember-sortable behaves.

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