Limit SE and HE editor assignment dropdown options to people with those permissions
Senior Editors and Handling Editors are discrete lists, and not everyone should be able to be assigned as an SE or HE.
To prevent users being assigned an incorrect role on a manuscript, the Senior Editor assignment drop-down should show only the subset of users who have Senior Editor permissions, and the Handling Editor assignment drop-down should only show the subset of users who can act as Handling Editors.