Update url to the API docs throughout the Guides/Tutorial

[jenweber]

This issue has been solved and is ready for someone to work on!

Problem:
Right now, most links in the Guides that point to the API docs have a hard-coded version number in them, like https://emberjs.com/api/ember/2.16/classes/@ember%2Fapplication/methods/getOwner?anchor=getOwner. We need to update this, especially while getting ready for 3.0

Solution:
Thanks to some recent work in the API docs, now we can use a special url path called release that will point to the latest version, like https://emberjs.com/api/ember/release/classes/@ember%2Fapplication/methods/getOwner?anchor=getOwner. Making this change should theoretically be a careful find/replace of /2.16/ for /release/ in API urls found in the .md files of the Guides.

To do:

• try replacing the version with release in a few places to make sure it works as expected
• update the urls throughout the guides with find/replace
• Spot check the releaseified links to make sure they work
• review a diff to see if you made the changes you meant to, git diff master
• Read through this issue to see which api urls weren't able to be fixed because documentation was missing. Make those changes. There shouldn't be missing docs anymore.
• open a PR

Links were hard coded as a temporary measure in this past PR during the implementation of RFC 176.