Integrate successful accessibility features to be the default for new Ember apps

I'd like to see us integrate some of the successful accessibility improvements we implemented as addons so we can move to the next iteration of improving accessibility in Ember.

We set a goal: "No new Ember app should fail accessibility success criteria" and we have made improvements to this goal, such as integration of page titles and `unique-id` helpers for template-only components, but I think we have a few polish items left.

1. **set `lang="en"` by default, and not a flag that the developer has to remember when creating a new app.** Rationale:
- Our survey data and community feedback says that not only are most Ember apps in English, apps in a different language do not mind having English set to the default.
- The only objection I have heard is from a few devs in the US that do not want to assume these things. I think that is considerate, but I also think in this one instance, it's okay to do this.
- Developers could still set it differently via flags (eg, you can use `--lang` today) when creating a new app.
- I am not suggesting that our data is perfect, only that it is the data that we currently have.
- We can work on improving our data (e.g., try to find community members or sponsors who are willing to help with survey translations) and periodically re-visit to ensure that the defaults are aligned with our community's use.
- When the RFC for interactive new Ember app creation is implemented, `lang` could be one of the options that developers are given.
2. **accessible routing and skip links**: integrate [ember-a11y-refocus](https://github.com/ember-a11y/ember-a11y-refocus), which gives us accessible routing by default in an Ember app.

I think there are some new approaches to accessibility in JS apps that Ember could demonstrate, and there's a lot of room for growth. First, however, I would like to see us really get these things over the finish line.