Add canonicalization to pages

As part of the SEO audit, it was found that we do not currently have canonical URLs for pages on the site. This is important to make sure that search engines don't put URLs with non-important parameters into the search index. You can read more about it here:

https://support.google.com/webmasters/answer/139066#2

We need to add canonical URLs into the head of every page, so this should be added to the default layout. The link should look like this:

<link rel="canonical" href="http://www.yoursite.com/url-for-page" />