[#3168] - save datapoints in api

[marvinkome]

Before the PR (what is the issue or what needed to be done)

Datapoints were only stored in memory and not saved in the data store.

The solution

Store datapoints with the API and fetch back datapoint assignments when editing assignments

Screenshots (if appropriate)

Reviewer Checklist

• Added an explanation about the work done
• Connected the PR and the issue on Zenhub
• Added a test plan to the issue
• Updated the copyright header (when relevant)
• Formatted the code
• Added a documentation (if relevant)
• Added some unit tests (if relevant)

[muloem]

Try to review the datastructures to ensure that whats contained in them is accurately reflected by the name. e.g. an array of Ids should have something referring to Ids in its name in order to distinguish it from an array of objects.

[marvinkome]

There's a bug where Datapoints isn't being saved because surveyAssignment keyId isn't yet available when creating new surveyAssignment, this bug was fixed in this PR #3404

[muloem]

When trying to assign the datapoints I get a blank screen and this error in the console.