Bugfix: console errors on unmount

[KoolADE85]

This PR fixes a flood of Javascript errors thrown when the player is unmounted while a video is playing.

While I'm in the neighbourhood:

• move CircleCI job to a Github action
• update dev dependencies to a modern stack
• fix trivial console errors around invalid React syntax:
  • use className instead of invalid class
  • use React.createRef() for refs instead of hard-coded string player

Fixes #74
Fixes https://github.com/plotly/ddk-dash-docs/issues/4081

[AnnMarieW]

Looks good!
Just curious - why is react-player not listed as a dependency in package.json

[AnnMarieW]

Also:
closes #78

[KoolADE85]

Looks good! Just curious - why is react-player not listed as a dependency in package.json

but it is! 😃 :

[AnnMarieW]

Looks good! Just curious - why is react-player not listed as a dependency in package.json

but it is! 😃 :

Oh, haha - What's it doing way up there? I always look for it near the devDependancies. I totally missed it! 🙈