Dynamic webhooks API allows to register webhooks to one url per app instead of per user

Issue Summary

There are two similar threads that appeared on the community recently and they are both related to similar stuff. 

Once the user-A registers webhooks to the URL-A, then the user-B can't register webhooks to URL-B and they can't display the URL used by the user-A

This is reproducible on Data Center: no

Steps to Reproduce

1. Create a 3 LO app that allows users to register the webhooks
2. Install the app on the instance as USER-A
3. Register a webhook to URL-A
4. Install the app as USER-B

Expected Results

1. The limit should take the user under consideration - allow one recipient URL per pair app & user.
   OR
2. Get all webhooks should return all registered webhooks for that app

Actual Results

1. You can't register a webhook to URL-B as USER-B
2. You can't list webhooks registered by USER-A, so you can't check the URL used by USER-A

Workaround

Currently, there is no known workaround for this behavior. A workaround will be added here when available