Show resolve fields only when conflict occur

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Minor fixes regarding conflicts

TODO: find out why in NestedModelAdmin you cannot totally hide fields
no matter if HiddenInput() is used while in InlineModelAdmin you can
(if you remove them from Meta.fields).

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>