Add `no-unsafe` rule

[sergei-startsev]

Certain legacy lifecycle methods are unsafe for use in async React applications and cause warnings in strict mode:

• UNSAFE_componentWillMount
• UNSAFE_componentWillReceiveProps
• UNSAFE_componentWillUpdate.

These also happen to be the lifecycles that cause the most confusion within the React community. At the same time there're no deprecation warnings regarding using UNSAFE_ methods and it will still be possible to use them in React 17.

The rule prevents usage of of UNSAFE_ lifecycle methods. See discussion details #1750 (comment).

[sergei-startsev]

The rule has been merged.