Disable email

**Is your feature request related to a problem? Please describe.**
Setting up emails can be problematic, and most users do not need it when they rely on an external mechanism for authentication.

**Describe the solution you'd like**
Have the possibility to disable email system entirely: no notification by mail for users.

The issue does not identify files, tests, or an entry point. Start by tracing the backend email and notification flow and its configuration; done means email can be disabled entirely so users receive no mail while external authentication remains usable.

Written by the indexing model from the issue text.