Adjust data and UX to account for differing ember-data versions

[jenweber]

Moving forward, ember-data may not have a release corresponding to every release of ember-source. We need to account for this in our UX and the data layer. This app is tightly tied to ember-jsonapi-docs, and work spans both repos.

To do:

• In ember-jsonapi-docs, generate two version files instead of one. One versions file for ember-data, one for emberjs
• In ember-api-docs, use the 2 separate version files to populate the dropdown, based on whether you are in emberjs or ember-data. Probably this will require changes to adapters/serializers
• Nice to have - create an error template for the experience if a version 404s. It would be great to still allow someone to interact with the versions dropdown and recover, but this is not necessary.
• When writing new logic, if convenient, please do not assume that versions are numbers. Someday we dream of offering "canary" but this is just a dream, so don't spend extra time to make this possible. It's just if you are making decisions where it's equally as easy to not assume numbers.