feat: admin editable email templates

addresses #869eg7ptr

templates/email/generic.html.twig

@@ -0,0 +1,15 @@
+{% extends 'email/layout.html.twig' %}
+
+{# Every string on `text` was escaped by App\Email\EmailTextRenderer before the <strong>,
+   <p> and <a> tags in it were introduced, which is what makes |raw safe here. Do not
+   pass anything into this template that did not come out of that renderer.
+   The closing button is rendered by the layout. #}
+{% block body %}
+    {% if text.headline is not empty %}
+        <h1>
+            {{ text.headline | raw }}
+        </h1>
+    {% endif %}
+
+    {{ text.bodyHtml | raw }}
+{% endblock %}