use defaultChannelSlug in queries and mutations

[AlicjaSzu]

I want to merge this change because...

• Store the channel slug in the storefront
• Update queries that require channel slug and pass the value to these queries to get channel’s data.
• Update featured products - use products from channel with featured-products slug

Screenshots

Pull Request Checklist

1. [x] All visible strings are translated with proper context.
2. [x] All data-formatting is locale-aware (dates, numbers, and so on).
3. [x] The changes are tested.
4. [ ] The code is documented (docstrings, project documentation).
5. [x] Changes are mentioned in the changelog.

Test Environment Config

API_URI=https://master.staging.saleor.rocks/graphql/

[patrys]

Task linked: SALEOR-458 Use Channel SLUG in storefront

[jwm0]

⚠️ This PR uses temporary non-released SDK version that should not be merged into master - commit 1fb1a01

Now using published version @0.3.0-0 👍

[krzysztofwolski]

Waiting for stage server update, then tests should pass

[patrys]

Task linked: SALEOR-1645 Drop deprecated navigation field from shop query