Show more reasonable ID in a CICI link's text representation

With respect to a classification root node, class_instance_b of a
ClassInstanceClassInstance (CICI) link is the actual root class instance
of a classification graph. Its class_instance_a would refer to the
linked project. Since there can be many CICI links to the actual root,
but from different projects, it is makes sense to not show the link's
ID, but the root's.
Importer: change style to fit more into admin layout

This commit also changes the user and group permission control elements.
Now the FilterSelectMultiple class is used there, which is in use on
other places in the Djange admin. This increases usability for multiple
selections on large lists.
Importer: allow manual selection of classification graphs to link

If a user chooses to link classification graphs on the first page, (s)he
now able to not only linke the suggested classification graphs, but also
any graph wanted.
Importer: increase width of classification graph control
Merge branch 'importer-update'