fix: set preferred email in Employee via backend controller

[gavindsouza]

Noticed when Employee was bulk updated from list view for prefered_contact_email to "Company Email" did not set the appropriate values under preferred_email because the respective code was in the desk script 😄

Set "Preferred Email" for Employee via validate based on "Preferred Contact Email" value which can be either "Company Email", "Personal Email" or "User ID".

Additional change to note: Unset preferred_email when prefered_contact_email is also unset.