Release 6.20.0: Disable contact picture in mail view

[changerequest]

Showing the contact picture in mail view requires an email address based search. This might cause load on some backend, esp. those which directly connect to LDAP. Therefore, admins need a configuration to disable this feature.

New configuration options:
ui/mail/showContactImage/value = true/false
ui/mail/showContactImage/configurable = true/false