We're only calling the `block_for` method from one place: the
`content_block` helper, and we're never passing the locale parameter to
that helper, which means we're always calling `block_for` using
`I18n.locale`.
So I'm not sure why we were doing the `locale ||=` assignment, since
`I18n.locale` doesn't return nil.
In any case, since this was added in commit c1de2dced, Ruby has added
support for arguments forwarding, so we can use it here to simplify the
code a little bit.
722e50a669