Changes default text to "Unassigned"

By changing this text the css will check for `toggle_text == default_label`. In this case they were not the same.
Now they are and it will show the default label in gray.

app/helpers/form_helper.rb

@@ -29,7 +29,7 @@ module FormHelper
         current_user: true,
         project_id: issuable.project.try(:id),
         field_name: "#{issuable.class.model_name.param_key}[assignee_ids][]",
-        default_label: 'Assignee',
+        default_label: 'Unassigned',
         'max-select': 1,
         'dropdown-header': 'Assignee',
         multi_select: true,