Fix submit button text for Admin Notifications admin form

In Madrid, the button text didn't change depending on whether the form
is for the "new" page or for the "edit" page.

In consul, the buttons texts were "create admin notification" and
"update admin notification" instead of "create notification" and "update
notification".

Also change translation key from "submit" to "submit_button" to
match other instances.

app/views/admin/admin_notifications/_form.html.erb

@@ -8,6 +8,7 @@
   <%= f.text_area :body %>
 
   <div class="margin-top">
-    <%= f.submit class: "button success" %>
+    <%= f.submit t("admin.admin_notifications.#{admin_submit_action(@admin_notification)}.submit_button"),
+                 class: "button success" %>
   </div>
 <% end %>