fix: load default address in checkout

[lukaszjedrasik]

Related Issues

closes #4682

Short Description of the PR

If shipping data are available for this account, the shipping form will be filled automatically.

Screenshots of Visual Changes before/after (if There Are Any)

Default theme:
https://user-images.githubusercontent.com/40273258/115393821-6559d900-a1e2-11eb-8a1a-0ac55c1b0ce3.mp4

Capybara theme:
https://user-images.githubusercontent.com/40273258/115393892-79053f80-a1e2-11eb-857c-e95d3e841843.mp4

Pull Request Checklist

• I have updated the Changelog (V1) v2 and mentioned all breaking changes in the public API.
• I have documented all new public APIs and made changes to existing docs mentioning the parts I've changed so they're up to date.
• I have tested my Pull Request on production build and (to my knowledge) it works without any issues

• I have followed naming conventions

[Fifciu]

It requires solid manualy testing. I am afraid this watcher will be invoked on every step of the checkout as Shipping.vue is mounted all the time on the checkout (at least it was)