[Backport] Fixing the customer subscribing from different stores

[eduard13]

Original Pull Request

#19195

Description (*)

This PR fixes the customer subscription from different stores.

Fixed Issues (if relevant)

1. Newsletter subscription does not set the correct store_id if already subscribed. Not Fixed in 2.3-dev #19172: Newsletter subscription does not set the correct store_id if already subscribed

Manual testing scenarios (*)

1. Create 2 stores: A(default) and B
2. Login on the frontend
3. Subscribe to Newsletter (which should be successfully)
4. Switch to store B
5. Subscribe to Newsletter (you get the success message, but the subscription wasn't saved)

For details, please check the task's description.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @eduard13. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me 2.2-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3579 has been created to process this Pull Request

[magento-engcom-team]

Hi @eduard13. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.

[Ctucker9233]

@magento-engcom-team This was not included in 2.2.8. Will it be in 2.2.9?