Add sort order to multirow instances

Summary

1. As a configurator

2. I want insert sortOrder value to multirow instances when the multirow field has multirowField.meta.options.reorderAllowed: true option

3. so that multirow instances can be reordered

Acceptance Criteria

1. when a multirow field has reorderAllowed: true, the related data that populates the multirow should be sorted by the sortOrder property see Add generic "sortOrder" property to instances #1839

2. when a multirow field is saved, the order that the multirow array values are PATCHed will determine the value of the sortOrder property, so if it is PATCHed like this:

personName: Lewis
favColours: [
  {
    id: 1
    name: red
  }
  {
    id: 2
    name: green
  }
]

then we will save these 2 to Colour model db:

  {
    id: 1
    name: red
    sortOrder: 0
  }
  {
    id: 2
    name: green
    sortOrder: 1
  }