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.
I think this macro should really be named `related documents` and that we should only set a default on `quantity`. Seems weird to set a default for `doc_types` if `post` can be any doc type.

This would also mean adding a parameter for `heading_label`.