fix template preview

user did not have access to preview template (when looking at screen in backend) if concerto was not in public_concerto mode
allow empty in screen field configs

if screen field has no FieldConfig's yet, access to the field config list is denied, leading to a redirection when user want's to add new field configs
Merge pull request #1066 from simplysoft/fix-template-preview

fix template & field config permissions