PAGES context variable is replaced by pages in Pelican 3.7

By using 'or' in the if statement, we can support Pelican 3.7 and
earlier versions in the same theme. When we are ready to drop support
for Pelican <3.7, the if statement can be simplified.

See the following links for more information.

http://docs.getpelican.com/en/3.7.0/changelog.html#id1

http://docs.getpelican.com/en/3.7.0/faq.html#since-i-upgraded-pelican-my-pages-are-no-longer-rendered

templates/base.html

@@ -120,7 +120,8 @@
                     {% endfor %}
 
                     {% if DISPLAY_PAGES_ON_MENU %}
-                        {% for p in PAGES %}
+                        {# FIXME: Simplify to 'for p in pages' when we are ready to drop support for Pelican < 3.7 #}
+                        {% for p in PAGES or pages %}
                             <li{% if p == page %} class="active"{% endif %}><a href="{{ SITEURL }}/{{ p.url }}">{{ p.title }}</a></li>
                         {% endfor %}
                     {% else %}