Handler::VNEdit: Use json_validate() for the VN staff/cast data
Handler::VNEdit: Use json_validate() for the VN staff/cast data

This (only very slightly) simplified further processing of the data. It
does add more validation than was present: Previously it was possible to
send invalid roles (would give a 500) or invalid numeric IDs (would
throw some perl warnings). These issues are now handled earlier on. This
change also puts a maximum length on the notes field, but nobody has
passed the 300 characters so far.

christian-roggia/vndb,christian-roggia/vndb,morkt/vndb,morkt/vndb,erengy/vndb,erengy/vndb,morkt/vndb,christian-roggia/vndb,erengy/vndb,morkt/vndb,morkt/vndb