json / CSV links are broken in Datasette 0.50

e.g. on https://latest.datasette.io/fixtures/sortable

That export link block is  broken. The HTML is:

```html
<p class="export-links">
  This data as <a href="//fixtures/sortable.json">json</a>,
  <a href="//fixtures/sortable.csv?_size=max">CSV</a> (<a href="#export">advanced</a>)
</p>
```