Add consumer-based RfC Visibility settings

This setting will be useful to increase data protection, where users might not be allowed to see RfCs from other contexts.

app/views/consumers/_form.html.slim

@@ -9,4 +9,7 @@
   .mb-3
     = f.label(:oauth_secret, class: 'form-label')
     = f.text_field(:oauth_secret, class: 'form-control', required: true)
+  .mb-3
+    = f.label(:rfc_visibility, class: 'form-label')
+    = f.collection_select(:rfc_visibility, Consumer.rfc_visibilities.map { |rfc_visibility, _id| [t("activerecord.attributes.consumer.rfc_visibility_type.#{rfc_visibility}"), rfc_visibility] }, :second, :first, {}, class: 'form-control form-control-sm')
   .actions = render('shared/submit_button', f: f, object: @consumer)

app/views/consumers/show.html.slim

@@ -6,5 +6,6 @@ h1
 - %w[oauth_key oauth_secret].each do |attribute|
   = row(label: "consumer.#{attribute}") do
     = content_tag(:input, nil, class: 'form-control bg-secondary', readonly: true, value: @consumer.send(attribute))
+= row(label: 'consumer.rfc_visibility', value: t("activerecord.attributes.consumer.rfc_visibility_type.#{@consumer.rfc_visibility}"))
 
 = render('study_groups/table', study_groups: @consumer.study_groups.sort)