Use hints instead of placeholders in URL fields

Placeholders are hard to read due to their low contrast, and users might
want to read them after they're already gone, and so they cause both
accessibility and usability issues.

app/components/admin/budget_phases/form_component.html.erb

@@ -51,7 +51,7 @@
     </div>
 
     <div class="small-12 column">
-      <%= translations_form.text_field :main_link_url, placeholder: t("admin.shared.example_url") %>
+      <%= translations_form.text_field :main_link_url, hint: t("admin.shared.example_url") %>
     </div>
   <% end %>