Context menu to Link to Viewport

[will-moore]

This adds the "get link to current viewport" that we have in the old webgateway viewer, including the newly-fixed x, y, and zoom parameters to link to the current location AND rendering settings.
Card: https://trello.com/c/PuziBOFJ/5-image-link-feature

To test:

• Pan, zoom and update rendering setting on an image without saving (include quantization family etc)
• Right-click on image -> Get Link to Viewport
• Copy URL and check that by pasting it in a new window, all settings are preserved:
  • Channel colours, on/off, ranges, quantization family, coefficient, inverted etc.
  • Zoom and X and Y coordinates of the viewport are respected
• Also, if the original URL had e.g. dataset=1, check that this is included in the new URL (image in multiple Datasets should preserve which Dataset is shown in thumbnail panel).

[manics]

LGTM