Ember Octane (and other updates that we have to do)

[guidojw]

Here is a list with everything that we have to update in order to be fully using Ember Octane,
I also included some other updates because of feature deprecations/removals here.

TODO: see list of sub-issues. More sub-issues may be added later

Links to refer to for updates, guides & deprecations

• https://guides.emberjs.com/release/upgrading/current-edition/ & https://ember-learn.github.io/ember-octane-vs-classic-cheat-sheet/
• https://guides.emberjs.com/release/upgrading/
• https://emberjs.github.io/rfcs/
• https://deprecations.emberjs.com/
• https://github.com/ember-cli/ember-cli/releases
• https://blog.emberjs.com/tag/releases & https://github.com/emberjs/ember.js/releases
• https://blog.emberjs.com/tag/ember-data & https://github.com/emberjs/data/releases

[DrumsnChocolate]

#902 is a nice-to-have, but I've added it as a sub-issue anyway