style: make edit sponsor info link a button

Uses the PSF site `button` class instead of a plain text link
for better visibility and consistency with the rest of the UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JacobCoffee

apps/users/templates/users/sponsorship_detail.html

@@ -33,7 +33,7 @@ <h3>Sponsor: {{ sponsor.name }}</h3>
                     <li><b>State:</b> {{ sponsor.state }}</li>
                     <li><b>Country:</b> {{ sponsor.country }}</li>
                 </ul>
-                <small><a href="{% url 'users:edit_sponsor_info' sponsorship.sponsor.pk %}">Edit sponsor information</a></small>
+                <a href="{% url 'users:edit_sponsor_info' sponsorship.sponsor.pk %}" class="button">Edit sponsor information</a>
             </div>
 
             <div id="application-info" class="card small-column">