Separate config option to disallow email changes by users

[akhil1508]

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Is your feature request related to a problem? Please describe.

• As an admin of a nextcloud server, I'd like to disallow users from changing/updating their email
• Currently it is coupled with this allow_user_to_change_display_name setting
• I would like to allow them to change their name but not their email i.e. assign them an email at the beginning that cannot be updated later on

Describe the solution you'd like

• A config option of allow_user_to_change_email to (dis)allow user to update email
• Can default to true

Describe alternatives you've considered

• Patching the frontend to make the email field readonly 🤷‍♂️

[kesselb]

Closing your report in favor of #16823