Strip `<p>` tags from headers in revealjs slides

When the template renders a revealjs page, the slides look something like this (truncated for brevity)

<section>
  <h2>
    <p>Slide Title</p>
  </h2>
  <!-- slide content -->
</section>

This is valid HTML, but it makes styling slides a bit difficult. @gus asked for help styling slide text, and the selector ended up being .reveal section > p, .reveal section > div because the usual .reveal p also styled headers. A quick jinja macro to strip the tags might be something like the following:

{% macro strip_p_tag(text) %}
  {{- text[3:-4] }}
{% endmacro -%}