big_image_link_xy_11844 (rebased onto dev_5_0)


This is the same as gh-1926 but rebased onto dev_5_0.

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This fixes the "Image Link" creation and processing, so that you can 'bookmark' a location and zoom level on a Big Image (see https://trac.openmicroscopy.org.uk/ome/ticket/11844).

To test, 
 - open a Big image in web viewer
 - pan and zoom to a chosen location (note zoom and remember what feature you're looking at)
 - copy "Image Link" from left of window
 - paste Link into another window (or another browser - login if needed) and check that you are directed to same location and zoom level.
 - Regression test - check that simply loading image (without following link) loads at centre of image and zoomed out.