Add scale bar

[nchuynh]

Add a scale bar to the image viewer

[kruegert]

This is the scalebar that is used in minerva story: https://github.com/usnistgov/OpenSeadragonScalebar
Which can then be drawn directly in the seadragon panel instead of aside (as in the example above).

[kruegert]

there is a function in our client-side data layer to get metadata, which should probably include the scale information (width heights in micros from the dataset, that can be used to calculate microns in viewport from pixels in viewport).
There should be code for this conversion in the scope2screen repository (check where the metadata call is called and used):
https://github.com/labsyspharm/scope2screen

[kruegert]

here is a screenshot on how it looks in minerva story: