Refactor widget templates to share more code

[simonw]

And document the new pattern.

[simonw]

New big_number-label.html:

{% extends "django_sql_dashboard/widgets/_base_widget.html" %}

{% block widget_results %}
  {% for row in result.rows %}
  <div class="big-number">
    <p><strong>{{ row.label }}</strong></p>
    <h1>{{ row.big_number }}</h1>
  </div>
  {% endfor %}
{% endblock %}