[Tech] Make it possible to disable the React developer tools

[CommandMC]

This PR allows for disabling the installation of the React dev tools by setting the HEROIC_NO_REACT_DEVTOOLS env var before running yarn dev.

I'm currently developing on a somewhat older machine, and I've noticed that this leads to just a little bit faster load times. And that little time goes a long way when you're restarting Heroic ~50 times a day :^)

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[arielj]

Looks good, would be good to update the README to show this next to the yarn dev instructions.