All extra_head blocks now call super

This means you can provide a custom base.html template that populates
extra_head and any of the default child templates will still render content
you included in that block.

Refs #158

datasette/templates/database.html

@@ -3,6 +3,7 @@
 {% block title %}{{ database }}{% endblock %}
 
 {% block extra_head %}
+{{ super() }}
 {% include "_codemirror.html" %}
 {% endblock %}
 

datasette/templates/query.html

@@ -3,6 +3,7 @@
 {% block title %}{{ database }}{% if query and query.sql %}{{ query.sql }}{% endif %}{% endblock %}
 
 {% block extra_head %}
+{{ super() }}
 {% if columns %}
 <style>
 @media only screen and (max-width: 576px) {

datasette/templates/row.html

@@ -3,6 +3,7 @@
 {% block title %}{{ database }}: {{ table }}{% endblock %}
 
 {% block extra_head %}
+{{ super() }}
 <style>
 @media only screen and (max-width: 576px) {
 {% for column in columns %}

datasette/templates/table.html

@@ -4,6 +4,7 @@
     {% if human_filter_description %}where {{ human_filter_description }}{% endif %}{% endblock %}
 
 {% block extra_head %}
+{{ super() }}
 <style>
 @media only screen and (max-width: 576px) {
 {% for column in display_columns %}