Fixed userResource request to get all users

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Fixes: #12471

Description

This PR fixes the request usersResource.getUsers that you can issue from the backoffice in angularjs. It's not actually used in the backoffice code, but could be used in packages or custom backoffice extensions.

There were two issues:

• The model binding of the request wasn't correct. I've aligned it with similar GetByIds requests in other controllers so it works now.
• The authentication wasn't correctly checking the querystring value for ensuring that only admin users can retrieve admin user details.

To Test:

• Throw the following code in somewhere where you have or have added an injected userResource:

      usersResource.getUsers([-1]).then(function (data) {
        console.log(data);
      })

• Check that it works correctly for an admin user but doesn't for a non-admin one.

[Migaroez]

Works as expected, code looks good and can't find any issues with the added requirement as the Handler early returns when the query string does not contain the specified parameter and there are no methods that use the policy where the "ids" parameter is used in a different way.