remove DeltaVision-specific constant from pixels service
create new disk usage request
implement new disk usage request
make new request callable

(add thumbnail service into request object factory registry)
adjust API of disk usage request
adjust disk usage implementation to match API change
Initial version of a prev/next button for the web viewer
Moving nextPrevImage() to fix merge conflict
Ignore keystrokes on other inputs, sliders etc
Next and Prev ignore hidden thumbnails
extend disk usage object with file counts
implement file counts for disk usage requests

refactors usage tracking to fit extra duties
Rename buttons to '< Prev' etc & semicolons
Disable Prev / Next if not available
Handle Prev / Next for search results and history page
Move Prev/Next buttons down below ROI count
Merge remote-tracking branch 'remotes/origin/develop' into trac-7240-disk-usage-count
Merge remote-tracking branch 'origin/develop' into prevNextWebViewer

Conflicts:
	components/tools/OmeroWeb/omeroweb/webgateway/templates/webgateway/viewport/omero_image.html
UI rearrangement

Moved color mode (rgb/grayscale) button and channel selection
buttons from the left-hand side of the preview to below
the preview.
Moved color picker out of the popup menu