Many site settings can be distructive or have huge side-effects
for a site that the admin may not be aware of when changing it.
This commit introduces a `requires_confirmation` attribute that
can be added to any site setting. When it is true, a confirmation
dialog will open if that setting is changed in the admin UI,
optionally with a custom message that is defined in client.en.yml.
If the admin does not confirm, we reset the setting to its previous
clean value and do not save the new value.