Token generation should be reachable using the old URL

With KNOX-2811, the token generation UI's URL is changed from '.../tokengen/index.html' to '.../token-generation/index.html'. The idea was that clients will use the Knox Home page to go to that particular UI. However, it might be the case that other 3rd party tools have that link hard-coded.
So it'd be nice to reach the same UI with an additional context with the old name (tokengen).