Allow links in table headers

Currently when a table header is sortable, the sort functionality is implemented via Javascript. I'd like to sort without javascript via a standard link to the server ala `/some_page/?sort=-col3`. Adjusting the HTML like this:
```html

Cores

```
does work but has the following problem: The th styles set the pointer to cursor: https://github.com/canonical/vanilla-framework/blob/564d150ebcb19faf1654fbc248b8182f24f59e49/scss/_patterns_table-sortable.scss#L30 giving the impression that the whole cell is clickable but the link covers only the text leading to a bad UX.

Not sure about how to fix this best.

Start with scss/_patterns_table-sortable.scss around line 30 and review how sortable table headers set the cursor and related styles. Check the supplied HTML link pattern, then verify that the link provides server-side sorting while the header's clickable-area styling no longer suggests that the entire cell is linked.

Written by the indexing model from the issue text.