Improve the behaviour of clicking "view in new version" button when new version doesn't have the same guide path

[mansona]

When viewing an old guide you always see a link to "view the current guide in latest version" at the top of the page:

If you click the link in a guide that has been deleted in the latest verison (i.e. the path is not valid any more) then you will see a 404 page error:

Interestingly, because this is not a 404 path and just the error template clicking back in the browser does not get you out of this situation 🙈 you need to instead refresh.

Steps to recreate

• start the appliation locally
• visit http://localhost:4200/v1.2.0/getting-started/editing
• click the VIEW v1.3.0 button
• 💥

Todo

• make this experience "better" for people
  • There is a full discussion of this problem in an issue on guides-source
  • this issue probably reuquires some experimentation and discussion with the @ember-learn/learning-core-team
• improve the experience of using the back button when this situation happens
  • this could be a guidemaker concern so might need to be done "upstream"
  • ember used to have the concept of eager transitions but it seems to have been removed in 1.12 🙈 (makes me feel old). Fixing this will require research and discussion
  • If this is too hard for now we can pull it out into a separate issue so it doesn't block the work 👍

[anotheredward]

My intended fix was to make this functionality consistent with the version picker from the previous guides app.
I implemented logic to avoid resetting the path when switching versions here:
https://github.com/ember-learn/guides-app/blob/master/app/controllers/version.js#L21

I'm not sure how this is handled in the new project, but it does appear to function similarly.

A discussion of that functionality can be found here:
ember-learn/guides-app#200

One consideration was making this redirect more global, so that any attempt to navigate to an invalid path (like the one that the button generates) would do a best-effort redirect, but I'm not sure about all of the consequences of that change.