Improve performance when editing custom texts

We were executing SQL queries searching for translations even for
records which were not persisted in the database and therefore had no
translations.

So we're now only looking for translations when the record is persisted.

Note that, when the record isn't persisted, we're using `I18n.translate`
instead of the `t` method. That's because the `t` method would also
perform database queries since we overwrite the `t` method in the
`i18n_translation` initializer.

On tabs with many records, like the proposals tab, requests are now up
to three times faster on production environments.

Tests related to this part of the application were failing sometimes
because requests took longer than `Capybara.default_max_wait_time`.
After these changes, the custom information texts pages load about 30%
faster when running the tests.

app/components/admin/site_customization/information_texts/form_field_component.rb

@@ -14,12 +14,12 @@ class Admin::SiteCustomization::InformationTexts::FormFieldComponent < Applicati
     end
 
     def database_text
-      if i18n_content.present?
+      if i18n_content.persisted?
         i18n_content.translations.find_by(locale: locale)&.value
       end
     end
 
     def i18n_text
-      t(i18n_content.key, locale: locale)
+      I18n.translate(i18n_content.key, locale: locale)
     end
 end