imagetransfer.py: ported to core (patch 6)

Ported imagetransfer.py to pywikibot core. Also fixed related import issue
in nowcommons.py, concerning 'nowCommonsMessage'.

Change-Id: I826aafe9d14e01abf01d5f6dfc947a33078d3689
Merge "imagetransfer.py: ported to core (patch 6)"