Fix regression from better "sortable" support (1.12)

Regression resulted in AttributeErrors when having the table template customized.
Use `view.sortable_class(th)` instead of `view.sortable_class(th['sort_index'])` in custom table templates.
Merge pull request #23 from 4teamwork/jone-fix-sortable-regression

Fix regression from better "sortable" support (1.12)