Add confirm dialog - delete selected item

By default, if you want to remove selected item from widget, confirm dialog will be showed.
To disable this, set ``plugin_options={'disable_confirm': True}`` in form field.
To customize text in confirm set ``plugin_options={'confirm_text': 'custom text'}`` in form field.

blocks https://github.com/allegro/ralph_assets/pull/315
if any thing will be fix in this p.req. change it to '} else'.