Proper handling of canonical URLs (#52)

* add canonical_url block to base.html, change canonical url for pages

* Avoid nesting block within block

That leads to repeating the block twice instead of replacing its value
from `base.html`

templates/page.html

@@ -2,6 +2,10 @@
 
 {% block title %}{{ page.title }}{% endblock title %}
 
+{% block canonical_url %}
+<link href="{{ SITEURL }}/{{ page.url }}" rel="canonical" />
+{% endblock canonical_url %}
+
 {# <!-- Choosing cover image --> #}
 {% if page.cover %}
   {% if page.cover|lower|truncate(4, True, '') == "http" %}