Add Author dropdown to PostSettingsMenu

Ref #3084
This PR does NOT hide the dropdown as required to close 3084.

- `EditorNewRoute` creates post with the author set to the current user
- added `authors` ArrayPromiseProxy (whoa, what?) to PSM
- added `changeAuthor` function that sets author and saves model when the user selects a new author
@javorszky Don't be hatin