InterfaceAgent; Fix "Invalid ChallengeToken"

The merge of the Object Manager branch caused a regression that caused
"Invalid ChallengeToken" errors, because the LayoutObject's intiialization
code ran in ->new(), but the information wasn't available
Code cleanup.
Merge branch 'interfaceagent-initialize-layout' of github.com:moritz/otrs
Use ObjectManager to create language object. Avoid double LayoutObject instantiation.
Fetch LayoutObject by OM in InterfaceCustomer.
Avoid double LayoutObject instantiation.
Merge remote-tracking branch 'upstream/master' into otrs-master