Add public-inbox to Email-In-Feature

- Adds the advanced option to accept email from non-users per category email-address
 - Adds tests covering the new feature
 - Adds UI to configure this feature in the frontend

app/serializers/category_serializer.rb

@@ -6,6 +6,7 @@ class CategorySerializer < BasicCategorySerializer
              :group_permissions,
              :position,
              :email_in,
+             :email_in_allow_strangers,
              :can_delete
 
   def group_permissions
@@ -40,4 +41,8 @@ class CategorySerializer < BasicCategorySerializer
     scope && scope.can_edit?(object)
   end
 
+  def include_email_in_allow_strangers?
+    scope && scope.can_edit?(object)
+  end
+
 end