Question bank: Category selector renders as nested form (invalid html)

In question bank, the current code for the category selector uses a single_select object, which is a form consisting of a single <Select> tag. Because this selector is already inside of the <form action="edit.php">, that means we have a form within a form, which is invalid. MSIE complains about the invalid html on its console.

To output valid HTML in a way that is consistent with the surrounding code, the linked commit replaces the single_select object with a html_writer::select.