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/translations/messages.fr.yml

@@ -84,3 +84,6 @@ Greater or equal than: Plus grand ou égal à
 Lesser or equal than: Plus petit ou égal à
 Precision: Précision
 Text after the field: Texte après le champ
+
+#custom field long choice
+Options key: Clé des options