Refactoring storeCodeFromRoute, resolve store code on SSR

[andrzejewsky]

Related issues

closes #3576

Short description and why it's useful

It solves the problem when we want to reveal store code on SSR.

Which environment this relates to

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility and no breaking changes)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

Contribution and currently important rules acceptance

• I read and followed contribution rules

[pkarw]

Looks nice :) the only question: how is It related to #3540? We should merge #3450 first in order to avoid conflicts and then this one

Cool, good job!

[andrzejewsky]

@pkarw I'm just started with reviewing this PR, it probably will be a conflict as we have modified same file (but not so complicated) so would be nice to merge it beforehand.

[pkarw]

Cool; @patzick was into CR of #3540 as well so please just sync with him and merge

[patzick]

Looks good and mostly moving code, so approving, but would be nice to add unit tests to there methods as for example they would throw an error in case of null param