As i know that adding the interface to the `StaticContent` would mean to add the whole dependency "SeoBundle" to the ContentBundle. I am not shure what to do atm, cause @dbu said to do so in a outdated comment (https://github.com/symfony-cmf/cmf-sandbox/pull/243#discussion-diff-11115619) but the last mail in mailing list sounds as to add the SeoBundle to sandbox only.