Big images fs 9701 (develop)

This fixes the PanoJS big image viewer in Web to handle zoom factors other than 50% between levels. 

Currently the required scaling factors for zoom levels is not provided by the server (http://trac.openmicroscopy.org.uk/ome/ticket/9813) so it is hard-coded in the BlitzGateway https://github.com/will-moore/openmicroscopy/commit/4c07e3f1bea4001d4a581badc1c86378265fe19b

This PR should be put on hold awaiting #9813.

To test, browse an SVS image in 'develop'.
 - Zoom in/out, Pan around the image etc.
 - Check that viewer remains centered on the same spot
 - Check that thumbnail view and main viewport correspond