Adding default URL configuration with canonical URLs

We need a single URL structure across about.gitlab.com. Our pages resolve with a few different options and adding canonical URLs for our pages will tell crawlers which page we consider the official version.

Had a short call with @williamchia where this came up and he has an idea for how we could set a default canonical URL for every page on about.gitlab.com. 🎉

cc @lbanks

todo

• Update head.html.haml to include logic that looks for cannonical_path variable. If it exists, use it. If it does not exist set the canonical URL to the the current URL including a trailing slash.

e.g. about.gitlab.com/product should have a canonical URL or about.gitlab.com/product/