editor: fix edition statement saving problem.

The 'responsibilities' field from document 'edition statement' should be
optional in the editor form. This commit hides this field (if it's
empty) so, now, the user must choose to add this field (and fill a
value). Editing a document without 'responsibilities' is now allowed by
default.

* Closes #906

Co-Authored-by: Renaud Michotte <renaud.michotte@gmail.com>

rero_ils/modules/documents/jsonschemas/documents/document-v0.0.1.json

@@ -524,9 +524,15 @@
             "items": {
               "title": "Responsibility",
               "$ref": "#/definitions/language_script"
+            },
+            "form": {
+              "hide": true
             }
           }
-        }
+        },
+        "required": [
+          "editionDesignation"
+        ]
       },
       "form": {
         "hide": true,