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.
Martin Brennan