The selection should be done regarding the list of cnm_folders, and not
the users assignments (the list of images will contains duplicates
otherwise, one image per user assigned to a folder, instead of once per
image).

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.