fix leaderboard through id not date

templates/leaderboard.html

@@ -51,7 +51,7 @@
                 <select id="through_session" name="through_session" class="session-filter-select" onchange="this.form.submit()">
                     <option value="" {% if not selected_session_date %}selected{% endif %}>Latest session</option>
                     {% for session in available_sessions %}
-                    <option value="{{ session.session_date }}" {% if selected_session_date == session.session_date %}selected{% endif %}>{{ session_label(session) }}</option>
+                    <option value="{{ session.session_id }}" {% if selected_session_id == session.session_id %}selected{% endif %}>{{ session_label(session) }}</option>
                     {% endfor %}
                 </select>
                 {% if selected_session_date %}