OPAC Holds table needs header scopes instead of sr-only text

[Stephanie Leary]

In the OPAC account holds table, each cell contains a span with extra text for screen readers (<span class="sr-only">Title</span>, for example). This is unnecessarily verbose. Instead, the corresponding <th> tag should have a scope attribute, like <th scope="col">Title</th>

Each row should also have a <th> tag for the checkbox cell with a scope="row" attribute. However, these header cells will be useless unless the checkboxes have proper labels; see bug 2043042.

See the "Scoping rows and columns" section of the MDN <table> reference for a complete example: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/table#scoping_rows_and_columns