Extend "liferay/react" variant in Clay

[wincent]

As described in my comment on liferay/clay#1648 we should be able to create a "react.js" file that allows you to extend "eslint-config-liferay/react" instead of "eslint-config-liferay" in projects like Clay. I believe this is possible because I've seen the eslint-config-airbnb project offer multiple entry points.

• Set up "react" variant (TBD: decide whether to create separate "typescript" variant as well).
• Use variant in Clay.
• Remove explicit plug-ins and rules from Clay that got rolled into the variant.

[wincent]

Corresponding internal ticket: https://issues.liferay.com/browse/IFI-557

[wincent]

Would be "closed" by #42 but we still need to synchronize this with Clay so I am re-titling it to make clear what needs to be done.

[wincent]

Clay PR is up: liferay/clay#2536

Going to close this one; anybody interested in following along can just subscribe over there.