Add rel canonical to derived copies

I have added code to base.coffee that checks to see if the json returns a parentId. If so, it is a derived copy and a rel='canonical' tag with the parentId is added to the head of the page. If not, no tag. 
`if links?` is translated to `if (typeof links !== "undefined" && links !== null) {`

It's not necessary to add test specifically against just null and undefined unless you really mean it (in this case, any other falsey code will be evaluated as true).

Also, an empty array is not falsey in JavaScript, and `querySelectorAll` will always return an array.  So in this case, both `if links?` and `if links` are both inadequate, because `!![] === true`.

I think what you're looking for is `if links.length` (you don't even need to explicitly check if it's `> 0`, although you can if you want to be more explicit.