Extract partial to render flag actions

The main obstacle to extract this partial was probably the paths for the
flag and unflag actions.

Now that we use Rails 5.1 `resolve` method to handle nested resources,
we can use `polymorphic_path`.

Also note the code is a bit ugly because comments render a divider. We
should probably use a CSS border instead.

Co-Authored-By: taitus <sebastia.roig@gmail.com>

app/views/admin/debates/show.html.erb

@@ -21,8 +21,8 @@
     <%= link_to t("debates.show.comments", count: @debate.comments_count), "#comments" %>
     <span class="bullet">&nbsp;&bull;&nbsp;</span>
     <span class="js-flag-actions">
-            <%= render "debates/flag_actions", debate: @debate %>
-          </span>
+      <%= render "shared/flag_actions", flaggable: @debate %>
+    </span>
   </div>
 
   <%= auto_link_already_sanitized_html wysiwyg(@debate.description) %>