fix login errors with mailchimp

[gibkigonzo]

Related issues

closes #3612

Short description and why it's useful

Added silent mode for GET subscription request. Fixed function apply to event listener on 'user-after-loggedin' event (onUserPricesRefreshed didn't return function). Added checking if currentToken is falsy value, because it can be initially as empty string (this caused running user/refresh request with empty token). Also allow to synchronize cart in the background.

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 Changelog on how to port existing VS sites with this new feature

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and currently important rules acceptance

• I read and followed contribution rules

[pkarw]

@andrzejewsky please do a CR + plrease remember about the issue with products sync i raised - I mean error handling for adding product in the client's side + there is a lacking else for forceClientState for this case and also for a case when the product should be removed (all details in Slack)