Simplify EsHeaderComponent implementation.

[alexeykostevich]

• Update the component to use Ember Octane API.
• Remove the code for reseting expanded on CSS breakpoints for
  a more predictable behaviour and prevention of memory leaks.
• Add navbar-expanded class to simplify styles and reduce coupling
  with HTML structure.
• Fix accessibility warnings.
• Add additional tests.

[mansona]

wow thanks for the contribution 🎉 I had a quick glance and it looks mostly ok, although I'm not sure we can use @tracked quite yet because of the versions of apps that will be consuming the styleguide 🤔 I'll have to get back to you on that one.

I'm concerned merging this into the update branch because I would much prefer to merge this into master once update is merged and the website-redesign-rfc branch is merged. There is a PR that is currently blocking this whole process #301 but I'm hopeful that we can get it unblocked this week 👍

I just wanted to let you know that there will be a delay getting this merged but it will likely get merged once that delay is fixed 🎉

[alexeykostevich]

Thank you for looking at this and sharing the details, @mansona ! It sound great: there is no rush at all with this PR.

I'm all for getting the master branch ready first. Frankly, currently, it takes time for new newcomers to find the right place to start. #301 will make the repository even more contributor-friendly -- great job!

Please, let me know if any help from my side is needed.

[alexeykostevich]

@mansona , I've update the PR target to master. Please, let me know if you have any concerns about the PR.

[mansona]

@alexeykostevich can you rebase this instead of merging 🤔 the history seems a tiny bit out of whack

Let me know if you have any questions about the rebase 👍 I'm happy to help if you need it

[alexeykostevich]

That's a very good point! I've created another PR (#319) with a 1-commit history.