Use a button to moderate proposal notifications

As mentioned in commits 5311daadf and bb958daf0, using links combined
with JavaScript to generate POST (or, in this case, PUT) requests to the
server has a few issues.

app/views/admin/system_emails/preview_pending/_proposal_notification_digest.html.erb

@@ -30,11 +30,14 @@
       </div>
     </div>
 
-    <%= link_to t("admin.system_emails.preview_pending.moderate_pending"),
-                admin_system_email_moderate_pending_path(system_email_id: "proposal_notification_digest",
-                                                         id: preview.id),
-                method: :put,
-                class: "button hollow float-right" %>
+    <%= render Admin::ActionComponent.new(
+      :moderate_pending,
+      "proposal_notification_digest",
+      text: t("admin.system_emails.preview_pending.moderate_pending"),
+      path: admin_system_email_moderate_pending_path(system_email_id: "proposal_notification_digest", id: preview.id),
+      method: :put,
+      class: "button hollow float-right"
+    ) %>
   </div>
   <hr>
 <% end %>

spec/system/admin/system_emails_spec.rb

@@ -328,7 +328,7 @@ describe "System Emails" do
       visit admin_system_email_preview_pending_path("proposal_notification_digest")
 
       within("#proposal_notification_#{proposal_notification1.id}") do
-        click_link "Moderate notification send"
+        click_button "Moderate notification send"
       end
 
       expect(page).not_to have_content("Proposal A Title")