Improve the device assignment CRUD logic

[muloem]

We currently have survey/device assignments saved as SurveyAssignment entities in the backend. These map the list of forms to the list of devices to which the forms have been assigned. In the current setup the list of forms can contain forms from multiple surveys. However, in the new setup (started in #2796) we will have this list of forms coming from only one survey.

We will update the CRUD logic in the backend for SurveyAssignment entities to ensure that it conforms to this restriction. In order to avoid breaking the app function, we will not change the DeviceSurveyJobQueue entities but only ensure that they correctly correspond to existing assignments

In this issue, we update the CRUD to ensure that the assignment-device (DeviceSurveyJobQueue) entities are correctly generated in an optimal way when an assignment is created and updated. Also that the entities are correctly deleted when an assignment is deleted.

Whereas this issue was originally written to implement the restriction that forms in an assignment only come from a single survey, the implementation work done thus far improves on the way assignments are generated but does not yet implement this restriction, because it requires the frontend to first realise these changes. We will forward the restriction to a new issue (see #3147).

• This is related to the work in Datascript to transform existing SurveyAssignment entities to new assignments format #3107

[muloem]

Test Plan

It is covered by regression testing of the assignments functionality. It should behave the same.