I'm not sure why it isn't already done by foundation's form builder. It
doesn't make any sense to change an ID of a form field without changing
the `for` attribute of its label.
Using the block syntax to generate the label with a <span> tag inside
isn't necessary after upgrading foundation_rails_helpers. Before the
upgrade, we couldn't do so because the <span> tag was escaped.
We were monkey-patching FoundationRailsHelper::Formbuilder, which made
form customization difficult. We can inherit from it, which is the
standard way of extending what an existing class does, and make our form
the default one.
