Image testing with image-exporter electron runner

[etpinard]

... or in other words, bye bye old image server and (hopefully) bye bye imagetest docker container.

To try this out:

• pull down https://github.com/plotly/image-exporter in a directory adjacent to plotly.js (or change the path to image-exporter in the plotly.js package.json)
• npm i
• then, try npm run baseline and npm run test-image
• you also try npm run test-export but this one needs plotly-graph PDF with electron's printToPDF orca#6 to work properly.
• 🎉

What's new:

• electron based testing using image-exporter runner app
• no http requests are made, the runner app reads the JSON mocks directly from your file system
• a pure node-js image comparison routine using Mapbox's pixelmatch and pngjs. That means bye-bye gm and imageMagick

Results:

• much faster test runs. On my machine, npm run test-image runs in under 1:30 minute (N.B. that's over 400 mocks folks)
• Should work consistently across all machines.

TODO:

• if this works out well for everyone, delete:
  • all imagetest related things in tasks
  • test/image/assets/get_image_request_options.js

[etpinard]

Moved to #2615