Use model to establish sort params instead of DOM

This means that tables which have their sort params and directions specified in config (such as those on site activity dashboards) won't be overwritten due to the the lack of DOM properties.
Handle the case where no sort order is defined on model

For graph tables, no sort order is defined by default, so just leave the table as is.
Merge pull request #709 from alphagov/bugfix/table-sorting

Use model to establish sort params instead of DOM state