Automatically fill package or service when only one given

[ScreamingDev]

When editing an order you can create a (return) label.
While choosing for a carrier the options for service and package change dependend on each carrier.

Sometimes the new options for service or package only contain one entry.
To make it a bit easier for the customer we can preselect this single option.

• If there is only one option for service or package available we preselect this option.
• If there are more than one option we keep the current behaviour.

See #66 where it was first implemented.