owncloud/core

Bugfix: Decouple change email from 'allow_user_to_change_display_name' setting

Before this change, with setting 'allow_user_to_change_display_name' false,
the user was not able to change the mail address in Settings->Personal->General
via the webUI.

With this change, the setting 'allow_user_to_change_mail_address' has been
introduced and change mail address has been decoupled from  setting
`allow_user_to_change_display_name`.

'allow_user_to_change_mail_address' must be set explicitly to false, to take effect.

https://github.com/owncloud/core/pull/39288
https://github.com/owncloud/core/issues/35103