Add support for MENUITEMS and DISPLAY_PAGES_ON_MENU

With this commit, `.sidebar-nav` is renamed to `.sidebar-social`. Then a
new class `.sidebar-nav` is created and used for the menu items.  This
change more or less restores `.sidebar-nav` originally was in the
`hyde.css` file.

Since `DISPLAY_PAGES_ON_MENU` defaults to `True`, this commit will
modify the appearance of your site. To go back to the previous behavior,
add this to your pelicanconf.py:

    DISPLAY_PAGES_ON_MENU = False

References:
- <https://docs.getpelican.com/en/stable/settings.html#basic-settings>
- <https://docs.getpelican.com/en/stable/settings.html#MENUITEMS>

templates/sidebar.html

@@ -12,7 +12,21 @@
 			<p class="lead">{{ BIO }} </p>
 			<p></p>
 		</div>
-		<nav class="sidebar-nav">
+		{% if MENUITEMS or DISPLAY_PAGES_ON_MENU %}
+			<ul class="sidebar-nav">
+			{% if MENUITEMS %}
+				{% for title, url in MENUITEMS %}
+					<li><a href="{{ SITEURL }}/{{ url }}">{{ title }}</a></li>
+				{% endfor %}
+			{% endif %}
+			{% if DISPLAY_PAGES_ON_MENU %}
+				{% for p in pages %}
+					<li><a href="{{ SITEURL }}/{{ p.url }}">{{ p.title }}</a></li>
+				{% endfor %}
+			{% endif %}
+			</ul>
+		{% endif %}
+		<nav class="sidebar-social">
 			{% for name, link in SOCIAL %}
 				{% if name == 'email' %}
 					<a class="sidebar-nav-item" href="mailto:{{ link }}">