Add general resolver to importer

To match the indent style of this template, and retain the comment, consider:

                {% for row in panel.rows %}
                     <tr class="js-import-row">
                         {% for field in row.fields %}
                             {% if field.css_class %}  {# blank or 'error' or 'warning' #}
                                 {% include "importer/partials/resolve_popover.html" %}
                             {% else %}
                                 <td data-field-name="{{ field.name }}">{{ field.value }}</td>
                             {% endif %}
                         {% endfor %}