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
extend disk usage object with file counts
implement file counts for disk usage requests

refactors usage tracking to fit extra duties
Merge remote-tracking branch 'remotes/origin/develop' into trac-7240-disk-usage-count
Merge pull request #2907 from will-moore/preview_panel_like_insight

Preview panel like insight
making QA feedback URL configurable
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2835 (Update OmeroPy/src/path.py)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2895 (Reload groups after users added/removed)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2902 (OmeroPy: src flake8 (I))
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2919 (fix #7240: provide disk usage count)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2926 (OmeroCpp: Glob test sources)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2927 (OmeroCpp: Glob example sources)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2945 (Add a plugin for determining disk usage.)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2953 (Download spw original files)
merge develop -Dorg -Ijburel/openmicroscopy:qa-submit-new -Ssuccess-only: PR 2954 (min-max set)