User API: move to primary document format

[ErisDS]

This issue is a result of the API format discussion in #2362, and is part of a larger project to move our API towards the JSON-API format which is documented in the Epic: #2124. This is one step in the journey towards achieving the User JSON object format laid out in #2349.

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The equivalent task for Posts (#2580) has been done with the PR #2596 and should serve as an example of how to do this for Users.

The aim of this issue is to change the format of the user object in all requests and responses so that both the single user and user collection has the user object being contained in an array with the key users:

{
    users: [{...}]
}

This will involve changing the request format, changing the response format, updating all of the places in which the response is used, updating the tests and adding coverage where it is missing.

[ErisDS]

Would really appreciate if you would add docs to this (#2125) as you go! Thanks 👍

[javorszky]

Willdo