make return_path_email and set_return_path configurable on website and store scope as well

[mhauri]

Make the values return_path_email and set_return_path configurable in Website and Store scope.

Description (*)

By now the values system/smtp/return_path_email and system/smtp/set_return_path are currently only configurable on GLOBAL scope.

This makes it impossible to set these values if you have multiple websites with different domains and mail senders.

Manual testing scenarios (*)

1. Configure Multiple Websites
2. Try to set return path and return email on Website or Store scope

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)

[m2-assistant]

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[orlangur]

Note for QA: please consult with appropriate PO in order to evaluate such change.

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-4738 has been created to process this Pull Request

[sdzhepa]

✔️ QA Passed

Tested the ability to set "Return-Path Email" on a website level by next flow

Preconditions

1. Go to Admin > Stores > All Stores and configure two websites, like
   
2. Go to Admin > Stores > Configuration > General > Store Emails Addresses and configure different emails per each store, e.g.
   
3. Go to Admin > Stores > Configuration > Advanced > System and configure Return-Path Email per each store. like:
   
4. Go to Admin > Stores > Configuration > General > Web and configure Add Store Code to Urls = YES

Steps

1. Go to the first website (e.g. "http://magento.i1.loc/default") and create new customer1
2. Go to the first website (e.g. "http://magento.i1.loc/additional") and create new customer2
3. Verify that source of each email has correct **Return-Path
   
   **

[m2-assistant]

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