Add "select all rows" checkbox to table header in validation step

[thomasthesecond]

I think it would be helpful to add a checkbox to the table header on the validation step to select all rows. I know shift-click works to select multiple rows, but I'm not sure that's always obvious. Plus, I think there are other benefits to having a select all rows checkbox.

I have this working in a fork and could easily open a PR here if you'd like.

[benzler]

I would like this feature. Currently I have added it to my fork as well, but I think adding it to this repo would be beneficial.