Fix drag/drop for mobile

[cdcadman]

• https://medium.com/@deepakkadarivel/drag-and-drop-dnd-for-mobile-browsers-fc9bcd1ad3c5
• https://medium.com/@dmautomationqa/can-selenium-be-used-for-mobile-testing-924a53721f54

[cdcadman]

https://medium.com/@begunova/appium-web-app-testing-python-e4fa2c129668

[cdcadman]

I set up testing on Android Chrome, but the drag/drop test passes (#22). This needs to be reconciled with real world testing.

[cdcadman]

On mobile, a long touch should allow drag/drop, but instead it opens a context menu to do something with the image. It would be nice to set the test to have the same behavior.