Add a header_description block to the header template
This makes it easier to write flexible templates for contents.lr files written in markdown
For example, the donate-thank-you.html
template I'm working on is only doable with this MR:
{% extends "donate-empty.html" %}
{% block header_description %}
{{ this.header_description }}
{# ... #}
{% endblock %}
{% block donate_content %}
{# ... #}
{% endblock %}
The block still falls back to {{ this.header_description }}
as a default, so nothing will change for templates that don't use the new block.