staempfli/magento2-module-checkout-newsletter-subscription

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### Description
<!--- Provide a description of the changes proposed in the pull request -->

### Fixed Issues (if relevant)
<!--- Provide a list of fixed issues in the format magento/magento2#<issue_number>, if relevant  -->
1. #<issue_number>: Issue title
2. ...

### Manual testing scenarios
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1. ...
2. ...

### 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)