Sorting can't be focused/used via keyboard

[kulmann]

Steps to reproduce

1. With oC Web, login & get redirected to All files
2. Use the TAB key to reach the Name column sorting indicator
3. Use the TAB key once more

Expected behaviour

Have the next sortable column header focused

Actual behaviour

The checkbox of the first data row is being focused

Solution

I think it's necessary to wrap the full content of the th in a button instead of only using it for the sorting.