Make a decision about editorial owner and attributed author controls

Removing the built-in "Author" control (which maps directly to the `post_author` field) may not be as desirable as we originally thought because there can still be a need for Editor and Administrator level users to change this value.

Authorship provides a means of attributing multiple, different users to a post. The post author (from the `post_author` field) may or may not be in the list of attributed users. With Authorship active this field indicates the editorial owner of the post rather than the attributed author.

On the SW project there is no need for a user to be able to control this value, but on ING there could be.

Note that this change only affects Editors and Administrators.

## Options

1. Reinstate the built-in Author control
- Allows control over the editorial owner
- Complicates the UI, two different controls for selecting users
2. Introduce a combo UI for selecting attributed authors and the editorial owner in one control
- As suggested during one of our review calls
- New UI, could be better but will be less standard
- Avoids the need for two different controls for selecting users
3. Replace the built-in Author control with another instance of the `AuthorSelect` component for selecting only one user
- More consistent UI
4. Keep what we have now which is just one piece of UI for the attributed authors
- Simpler UI overall
- Doesn't allow controlling the editorial user

## Related

* #1
* #15