change(fastapi): remove the GET /logout route; replaced with POST

Had to add some additional CSS in to style a form button the same
as <a> links are styled.

Closes #188

Signed-off-by: Kevin Morris <kevr@0cost.org>

templates/partials/archdev-navbar.html

@@ -45,9 +45,12 @@
 
             {# All logged in users see Logout #}
             <li>
-                <a href="/logout?next={{ next }}">
-                    {% trans %}Logout{% endtrans %}
-                </a>
+                <form action="/logout" method="post" class="link">
+                    <input type="hidden" name="next" value="{{ next }}" />
+                    <button type="submit">
+                        {% trans %}Logout{% endtrans %}
+                    </button>
+                </form>
             </li>
         {% else %}
             {# All guest users see Register #}