Move newsletter routes form user.router.js to newsletter.router.js

[theBeginner86]

Feature request-

To move newsletter routes to newsletter.router.js (new-file)

Is your feature request related to a problem? Please describe. (optional)

A clear and concise description of what the problem is.

Describe the solution you'd like

Create new file newsletter.router.js
Add the appropriate function call in app.js for the moved routes.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Describe how this feature will be useful to our readers.

This will add scalability to our project and also make the code more readily and modular.

Additional context

I am GSSOC'21 participant.

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[theBeginner86]

/assign

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This issue has been assigned to @theBeginner86!
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