Add plaintext part to invoice mail

aleksis/apps/tezor/templates/templated_email/invoice.email

@@ -19,3 +19,17 @@
 <a href="{{ BASE_URL }}{{ invoice.get_absolute_url }}">{{ BASE_URL }}{{ invoice.get_absolute_url}}</a>
 {% endif %}
 {% endblock %}
+
+{% block plain_content %}
+{% blocktrans with number=invoice.number description=invoice.description %}
+Please find attached invoice number {{ number }} for {{ description }}.
+Please carefully read the PDF file concerning all payment details.
+{% endblocktrans %}
+
+{% if invoice.status == "waiting" %}
+{% blocktrans %}
+Please visit the following link to view and make the payment:
+{% endblocktrans %}
+{{ BASE_URL }}{{ invoice.get_absolute_url}}
+{% endif %}
+{% endblock %}