Select All table header (and checkbox)

[afercia]

Currently, there's no "Select All" checkbox in the table headers. The current rendered markup is: <th class="column-cb check-column"></th> Whether or not the checkbox is going to be added, the table header needs some semantics and accessibility improvements.

• no checkbox: then the table header shouldn't be a th element because it shouldn't establish a relationship with all the cells in that column; screen reader would announce "Select All" as the "title" of each checkbox in the rows, while they are intended to select only their own row. This cell should be a td, see https://core.trac.wordpress.org/ticket/32892 and https://core.trac.wordpress.org/ticket/31654

• checkbox: even if the checkbox is going to be added, the cell should be a td; the checkbox should be properly labeled

[rmccue]

For now, the checkboxes aren't actually useful, so there's no way to add a select-all checkbox. Will fix that in the future, but for now, we can change to td