add a custom field long choice basic

The `custom field long choice` aim to provide a way to deal with choices
with a big possibilities.

The `custom field long choice` allow :
- to persist different options in the database ;
- each option has a key, a text (translatable string), and eventually a
  parent, and an internal_key
- every key can be activate or not. If the parent is inactivated, all
  childs are inactivated
- the internal key have two purposes :
    - link to an external csv file, with their own key ;
    - add a special class to results, to allow custom layout.

Currently, the field exists, but some elements are missing :

- a script for CSV import
- possibility to select multiple items
- edition of options
- handle of option without parents
- tests are missing

Resources/views/CustomFieldsRendering/choice_long.html.twig

@@ -0,0 +1,11 @@
+{% if values|length > 0 %}
+    <ul class="custom_fields long_choice {% if values|length == 1 %}unique{% endif %}">
+        {%- for value in values -%}
+            <li class="long_choice_item long_choice-key-{{ value.key }} long_choice-ikey-{{ value.internalKey }} long_choice-parent-ikey-{{ value.parent.internalKey }}">
+                    <i class="fa fa-check-square-o"></i>&nbsp;{{ value.text|localize_translatable_string }}
+            </li>
+        {%- endfor -%}
+    </ul>
+{% else %}
+    <div class="custom_fields_choice long_choice empty">{{ 'None'|trans }}</div>
+{% endif %}