Make use of `include_on_features_page` under /products/feature-comparison

After the discussion in https://gitlab.slack.com/archives/C0NFPSFA8/p1515427827000150, make it so that using include_on_features_page: false for a feature, it won’t appear in https://about.gitlab.com/products/feature-comparison/.

Currently this is only used for https://about.gitlab.com/features/.

---

Example: Swimlanes is a feature not yet released, but @victorwu has already added it in features.yml:

swimlanes is not a feature currently, so it shouldn’t appear in the comparison page