Addons: add link previews selector field (#626)

Required by readthedocs/addons#554

Author: humitos

readthedocsext/theme/templates/projects/addons_form.html

@@ -109,6 +109,7 @@
       {% block addons_link_previews %}
         <div class="ui tab" data-tab="link-previews">
           {{ form.linkpreviews_enabled | as_crispy_field }}
+          {{ form.linkpreviews_selector | as_crispy_field }}
         </div>
       {% endblock addons_link_previews %}