Fix logic for adding email/sms subscriptions

[fadi-george]

Description

1 Line Summary

• checks for existing subscriptions before adding new ones for email and sms

Details

addSms and addEmail methods both do something like

   const newSubscription = new OSModel<SupportedModel>(
      ModelName.Subscriptions,
      subscription,
    );

so a new model key/id is made so creating new records in subscriptions table (edited)

Systems Affected

• WebSDK
• Backend
• Dashboard

Validation

Tests

Info

Checklist

• All the automated tests pass or I explained why that is not possible
• I have personally tested this on my machine or explained why that is not possible
• I have included test coverage for these changes or explained why they are not needed

Programming Checklist
Interfaces:

• Don't use default export
• New interfaces are in model files

Functions:

• Don't use default export
• All function signatures have return types
• Helpers should not access any data but rather be given the data to operate on.

Typescript:

• No Typescript warnings
• Avoid silencing null/undefined warnings with the exclamation point

Other:

• Iteration: refrain from using elem of array syntax. Prefer forEach or use map
• Avoid using global OneSignal accessor for context if possible. Instead, we can pass it to function/constructor so that we don't call OneSignal.context

Screenshots

Info

Checklist

• I have included screenshots/recordings of the intended results or explained why they are not needed

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Related Tickets

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This change is 

[jkasten2]

This language change is unrelated to the scope of the PR. Was this an intended logic change?

[jkasten2]

nit, the "... and deletes duplicate subscriptions" noted here doesn't describe the return value, but rather the side-effect of calling this function. Can we move this note to the function description. Lastly I would use the term "cleanup" rather than "delete" as it's a bit more specific here.