Replace www.gitlab.com with about.gitlab.com

Created by: cirosantilli

Our home has changed, let's change it in the app.

Mass refactor on md files:

git ls-files | grep -E '\.md$' | xargs perl -lapi -e 's/www\.gitlab\.com/about.gitlab.com/'

DRYed up on views through promo_host and promo_url helpers.