Allow downloading plotly images

[DonJayamanne]

For #7221

[codecov-io]

Codecov Report

Merging #9234 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9234      +/-   ##
==========================================
- Coverage    57.5%   57.48%   -0.03%     
==========================================
  Files         551      551              
  Lines       29432    29445      +13     
  Branches     4460     4462       +2     
==========================================
  Hits        16925    16925              
- Misses      11561    11574      +13     
  Partials      946      946

Impacted Files	Coverage Δ
...rc/client/common/application/webPanels/webPanel.ts	14.28% <ø> (ø)	⬆️
...ent/common/application/webPanels/webPanelServer.ts	77.94% <ø> (ø)	⬆️
src/datascience-ui/interactive-common/handlers.ts	4.54% <0%> (-6.57%)	⬇️

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[rchiodo]