fix: Fixed sending of emails to only new newsletter-subscribers.

[johnson-oragui]

Summary

This pull request fixes the constant ending of emails to users and visitors that subscribes to newsletters.

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Description

This pull request fixes the constant ending of emails to users and visitors that subscribes to newsletters. With this fix, only newsletter subscribers that are subscribing for the first time would receive a notification email.

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Related Issue (Link to issue ticket)

newsletter fix

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Motivation and Context

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How Has This Been Tested?

This has been tested with postman. The pytest for this already exists.

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Screenshots (if appropriate - Postman, etc):

USER SUBSCRIBE FOR THE FIRST TIME

​USER TRIES TO SUBSCRIBE AGAIN

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change) ​

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.