Redirect to master branch if branched guide is missing

[durden]

Currently we return 404 for a guide URL that includes a branch and the branch was removed. This makes sense because we technically couldn't find that guide. However, it would be more useful to redirect to the master branch of that guide with a flash message or something noting that the branch no longer exists.

For example, going to http://tutorials.pluralsight.com/ruby-ruby-on-rails/using-https-with-ruby-on-rails?branch=prtkgpt will return 404 and redirect to the homepage. This guide has since been published and all branches were integrated and removed. Therefore, the prtkgpt user's changes were either accepted or rejected. We could redirect this request to the canonical version of the guide with a message mentioning that.