Make the Project type custom fields sortable in issue lists
In Issue lists, it might be useful to sort on custom fields added by Extended fields.
As you may have noticed, custom fields of new types added by Extended fields are not clickable for sorting in the Issue lists.
I suggest to add some code to make the column title clickable for sorting on it, on columns for custom fields of Project type.
The patch to do so is attached to this issue. It was tested on CentOS, Ruby 2.0 and Redmine 2.5.2.