Add a Notify Me button to the product page when at least one variant is sold out.

gistfile1.txt

{% assign show_button = false %}
{% for variant in product.variants %}
    {% if variant.available == false %}
    	{% assign show_button = true %}
  {% endif %}
{% endfor %}

{% if show_button %}
  <img src="{{ 'notify_me.png' | asset_url }}" id="BIS_trigger">
{% endif %}