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! 🙈

[camdecoster]

Looks good except for the few things we discussed.

[camdecoster]

As we discussed, this file got regenerated as part of the build process, but it doesn't need to be updated for the bug fix. You could revert this change or update the Dash version in dev requirements to 4.4.1 and update the setup.py Dash version floor. There's also an issue with newer Dash that resulted in the style prop getting dropped from the docstring.

[KoolADE85]

All done in 7cb037b
I reverted the dev requirements back to v2.18 of Dash just to keep the scope from creeping up here.
We can do the extra work to bump everything else as part of a major version later.