Add a flatpickr plugin to attach 'id' attribute to input element

- As we use `altInput`, the original input is hidden. This plugin forward the `id` attribute of the original input to the input created on-the-fly and used by end user.

app/assets/javascripts/admin/all.js

@@ -39,6 +39,7 @@
 //= require flatpickr/dist/l10n/sv
 //= require flatpickr/dist/l10n/tr
 //= require shortcut-buttons-flatpickr/dist/shortcut-buttons-flatpickr.min
+//= require flatpickr/dist/plugins/labelPlugin/labelPlugin
 
 // spree
 //= require admin/spree/spree

app/assets/javascripts/admin/util.js.erb

@@ -20,7 +20,8 @@ $(document).ready(function() {
           }],
           label: "or",
           onClick: onClickButtons
-        })
+        }),
+        labelPlugin({})
       ]
   }
   window.FLATPICKR_DATETIME_DEFAULT = Object.assign(
@@ -39,7 +40,8 @@ $(document).ready(function() {
           }],
           label: "or",
           onClick: onClickButtons
-        })
+        }),
+        labelPlugin({})
       ]
     }
   );