size_check: move max sizes from inline to a remote JSON file
This pulls the image size data from a file (in the QA team's
fedorapeople space), instead of having it in-line. This will mean
I don't have to cut a new relval release every time someone wants
a max size changed. At the same time I'm bumping the max size for
Scientific to 5G, which will resolve #17 differently.
Signed-off-by: Adam Williamson <awilliam@redhat.com>