Code improvements in lib/JobOrders.php

[skrchnavy]

trafficstars

@mlespiau:

The design could be improved - this feature is adding if-cases in at least two places of the code when we could use polymorphism instead for a cleaner design. ... I believe we can fix these design issues once symfony becomes the new standard.

See https://github.com/opencats/OpenCATS/pull/209#issuecomment-289968062 for details