Issue Summary

If user is subscribed to any webhooks on StatusPage, then the API throws an error saying "The email, phone information, or endpoint provided is already subscribed to updates." if we try adding subscribers to any other mode. ** Where as if the same behaviour is replicated on the UI, it works just fine. 

Steps to Reproduce

1. Use Create Subscriber API to add a webhook subscriber. 
2. Try adding an email subscriber using the Create Subscriber 

Expected Results

Subscriber should be successfully added.

Actual Results

The below error is thrown:

"The email, phone information, or endpoint provided is already subscribed to updates." 

Workaround

Subscribers can subscribe to different modes via UI as a workaround.