Add <sup> tags around links to footnotes

Initially I just added a class to the `<a>` but occurred to me that
actual `<sup>` tags would be semantically better. So.

Tweaked the default CSS to make the `<sup>` tag's font-size `1em`
because otherwise they're `75%` and hard to read and even harder
to tap.

For #330

app/assets/_dev/css/elements/typography.css

@@ -69,3 +69,7 @@ blockquote {
     @apply border-l-2 border-blue-medium pl-8;
     @apply pl-8 py-4 my-8;
 }
+
+sup {
+    font-size: 1em;
+}

app/assets/_dev/css/pages/publication-front-page.css

@@ -76,4 +76,4 @@ ol.footnotes{
         float: right;
         font-size: 1.4rem;
     }
-}
+}

app/modules/content/models/pages.py

@@ -655,6 +655,7 @@ def _rewrite_anchor(self, block) -> None:
                 foot_num = self._footnote_index(link['href']) + 1
                 link.string = f"[{foot_num}]"
                 link['name'] = f"source-{foot_num}"
+                link.wrap(soup.new_tag("sup"))
                 block['value'] = str(soup)
 
     def _footnote_index(self, href: str) -> int: