Add copyright footer to gallery and photo pages

Renders a © 2000–<current-year> Ashik Salahudeen line under both the
index and each photo page. The year comes from time.Now().Year() at
render time so the range stays current without manual updates.

templates/index.html

@@ -42,5 +42,9 @@
       </div>
     </div>
   </main>
+
+  <footer class="site-footer">
+    © 2000–{{.Year}} Ashik Salahudeen. All rights reserved.
+  </footer>
 </body>
 </html>

templates/photo.html

@@ -28,5 +28,9 @@
       </div>
     </div>
   </div>
+
+  <footer class="site-footer">
+    © 2000–{{.Year}} Ashik Salahudeen. All rights reserved.
+  </footer>
 </body>
 </html>