Combine v8.dev stats with stats from old blog

[mathiasbynens]

(Continuing the discussion from #22 (comment).)

Ideally, we’d get the combined Analytics for the old URL structure + the new one. (We still use the same tracking ID.) Is this possible without overcomplicating things? What I do manually is e.g. for https://v8.dev/blog/10-years I’d search analytics for the page slug 10-years, which would also match the old URL https://v8project.blogspot.com/2018/09/10-years.html.

Some context used to be on the old V8 blog, but is now available under a new URL. For example, these two URLs refer to the same content (and the former now redirects to the latter):

https://v8project.blogspot.com/2018/09/10-years.html
https://v8.dev/blog/10-years

We're still using the same Google Analytics tracking ID, so it should be possible to get the page views for /blog/10-years as well as for /2018/09/10-years.html and just add them up.

There's a complete list of mappings from old URLs to new URLs: https://github.com/v8/v8.dev/blob/28dab9d3064fc38bbf5d5a32430e724e7503aeec/url-mappings.js#L15-L90 Maybe we could use that reverse mapping in devwebfeed? I.e., if a mapping exists for the current URL (e.g. https://v8.dev/blog/10-years), also look up the stats for the old one (e.g. https://v8project.blogspot.com/2018/09/10-years.html) and add them up to get the total view count.