Adds dependent restricted association to some models

fix #348

app/views/admin/campaigns/_form.html.haml

@@ -19,6 +19,6 @@
           = f.input :utm_term, label: 'Term', hint: 'Campaign keywords. E.g. marketing+conference+opensource'
           = f.input :utm_content, label: 'Content', hint: 'Used to track the different types of content that point to the same URL (A/B Test).'
         = f.inputs name: 'Targets' do
-          = f.input :targets
+          = f.input :targets, collection: @conference.targets
       %p.text-right
         = f.action :submit, as: :button, button_html: { class: 'btn btn-primary' }