Permit markup on prev/next session context buttons

Previously, if a button was customized to have child DOM elements, such as:
```html
<a href="..." data-context-href="..."><span>Text</span></a>
```

it would not receive the intended behavior.  Becuase the click target would be a child element (the span in this case), not the element with the required dataset attributes

Author: jcoyne

app/javascript/blacklight-frontend/search_context.js

@@ -1,6 +1,7 @@
 const SearchContext = (e) => {
-  if (e.target.matches('[data-context-href]')) {
-    SearchContext.handleSearchContextMethod.call(e.target, e)
+  const contextLink = e.target.closest('[data-context-href]')
+  if (contextLink) {
+    SearchContext.handleSearchContextMethod.call(contextLink, e)
   }
 }