If the user is an 'afis' user, the users_assigned_X variables should be
iterables (or check for None in the target.html page to ensure we dont
do a for loop over None (non-iterable variable)).

This does not include the functions to edit the list of users assigned
for a particular target.

Shall be refactored, along with the database structure

Set the two lists, one for the users which will search only the marks,
and one for the references.

Only show the marks related to the current segment in the afis segment
edit page. If a mark contains multiple zones, the image is displayed on
all related segments.