add new demo symfony files

app/templates/form/fields.html.twig

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+{#
+   Each field type is rendered by a template fragment, which is determined
+   by the name of your form type class (DateTimePickerType -> date_time_picker)
+   and the suffix "_widget". This can be controlled by overriding getBlockPrefix()
+   in DateTimePickerType.
+
+   See https://symfony.com/doc/current/form/create_custom_field_type.html#creating-the-form-type-template
+#}
+
+{% block date_time_picker_widget %}
+    <div class="input-group date" data-toggle="datetimepicker">
+        {{ block('datetime_widget') }}
+        <span class="input-group-addon">
+            <span class="fa fa-calendar" aria-hidden="true"></span>
+        </span>
+    </div>
+{% endblock %}
+
+{% block tags_input_widget %}
+    <div class="input-group">
+        {{ form_widget(form, {'attr': {'data-toggle': 'tagsinput', 'data-tags': tags|json_encode}}) }}
+        <span class="input-group-addon">
+            <span class="fa fa-tags" aria-hidden="true"></span>
+        </span>
+    </div>
+{% endblock %}

app/templates/form/layout.html.twig

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+{% extends 'bootstrap_3_layout.html.twig' %}
+
+{# Errors #}
+
+{% block form_errors -%}
+    {% if errors|length > 0 -%}
+        {% if form is not rootform %}<span class="help-block">{% else %}<div class="alert alert-danger">{% endif %}
+        <ul class="list-unstyled">
+        {%- for error in errors -%}
+            {# use font-awesome icon library #}
+            <li><span class="fa fa-exclamation-triangle"></span> {{ error.message }}</li>
+        {%- endfor -%}
+        </ul>
+        {% if form is not rootform %}</span>{% else %}</div>{% endif %}
+    {%- endif %}
+{%- endblock form_errors %}