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.

The 'enter' key on the username field should focus on the password
field, and not send the current form to validation.

This situation should not append, but we never know...