allow data-* form parameters to be used as <form> attributes. (#336)

Contrary to the previous `form.data.blueprints.form` extraction model,
blueprints-expansion is not enabled.

Eg: `data-data-foo@: '\Grav\Plugin\Admin::route'` would *not* create
 a `data-foo='http://...'` attribute.

templates/forms/default/form.html.twig

@@ -120,6 +120,12 @@
     class="{{ parent() }} {{ override_form_classes|trim }}"
   {%- endblock %}
 
+  {% block embed_form_custom_attributes %}
+    {% for k, v in blueprints.form.attributes %}
+      {{ k }}="{{ v|e }}"
+    {% endfor %}
+  {% endblock %}
+
   {% block embed_fields %}
     {{ override_inner_markup_fields_start|raw }}
     {{ override_inner_markup_fields|raw }}

templates/forms/layouts/form.html.twig

@@ -1,7 +1,8 @@
 <form
     {% block embed_form_core %}{% endblock %}
     {% block embed_form_classes %}{% endblock %}
+    {% block embed_form_custom_attributes %}{% endblock %}
 >
   {% block embed_fields %}{% endblock %}
   {% block embed_buttons %}{% endblock %}
-</form>
+</form>