Add an option to not send Talk message contents via email

[MegadoomerX]

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Feature request

Which Nextcloud Version are you currently using:
29.0.9

Is your feature request related to a problem? Please describe.
In the default configuration, Nextcloud Talk will send chat messages unencrypted via email.

This is a problem, because all Talk apps use some form of transport encryption to protect sensitive information and the web app uses encrypted webRTC, however sensitive info is then sent unencrypted via email if not read in a timely manner, vulnerable to interception by other parties: Man-in-the-middle attackers, email server hosts, etc.

Describe the solution you'd like
Add a global setting to prevent Talk messages (contents) to be sent via email. There appears to be no setting like this currently.

Describe alternatives you've considered
Sending encrypted emails is not viable for most users, therefore the only solution is to not sent Talk messages (contents) altogether.

[nickvergessen]

Before that we should add upstream an option so that apps can know if they do things to prepare an email or a normal notification