Introduce a new method for the defaultSenderName of mails

Fix #11662

@keros @DeepDiver1975 @PVince81 
They are both still injected, they are just not registered as a service anymore, as they are simple strings.
I copied that from the other controller definition in `core/application.php`:
https://github.com/owncloud/core/pull/11937/files#diff-1