[Data Views] Table layout: add aria-sort to column headings as appropriate

[andrewhayward]

What problem does this address?

Columns in the table layout can be sorted, but this is only communicated visually, and not semantically.

What is your proposed solution?

If a column is sorted, aria-sort should be set on the relevant th, with a value of ascending or descending as appropriate.

...
  <thead>
    <tr>
      <th>
        <button ...>Title</button>
      </th>
      <th aria-sort="ascending">
        <button ...>Author ⌃</button>
      </th>
      <th>
        <button ...>Status</button>
      </th>
      <th>
        Author
      </th>
    </tr>
  </thead>
...