Drop Ember Data and Ember Data Model Fragments in favor of native classes

[jelhan]

Ember Data and Ember Data Model Fragments add a lot complexity without providing much value. We should drop both in favor of native JavaScript classes for the models.

The Poll and User models should have a save method, which persists them server-side. The Poll model should have a load method, which loads a poll with all users from the server.

The load method should take care of parsing the response as well as decrypting the values. The save method should take care of serializing to as well as encrypting.

This will also unblock the upgrade to Ember v5.