[Feature] Email developers when/admin an invited user signs up

[areyabhishek]

Summary

1. We should email the admins, developers when a user signs up for their org or checks out an app for the first time in their org(users who already had an Appsmith account).
2. They should be notified so that they are aware of all the new users signing up and using the app they've built.
3. Viewers who've invited other viewers to Appsmith should get notified only when a viewer they invited signs up.

[Nikhil-Nandagopal]

@areyabhishek should we really do this? I can't recall a SAAS product that has notified me that the person I invited joined.