Better templates

Now that we've got a few different layouts under our belt it's time to make them more reusable. This PR allows us to start a new 2-1 layout as simply as this:

```jinja
{% extends "layout-2-1.html" %}

{% block content_main %}
    <h1>Main content</h1>
{% endblock %}

{% block content_sidebar -%}
    <p>Sidebar</p>
{%- endblock %}
```

Want one with a bleedbar?

```jinja
{% extends "layout-2-1-bleedbar.html" %}
```

There are other templates available which will need to be documented. Documentation is still to come.
Is it possible to give macro variables default variables? For example, it might make sense for `doc_types` to default to `posts,newsroom` and `quantity` to `5`, if they're not otherwise specified.