chore: migrate to react 18

[shadowusr]

What's done:

• React 16 -> React 18
• Enzyme -> @testing-library/react
• Updated a bunch of other packages
• Tests are mostly updated to testing-library without major changes, but some tests were refactored here and there. Notably, test/unit/lib/static/components/modals/screenshot-accepter/index.jsx was rewritten from ground up.
• rc-resize-observer -> @react-hook/resize-observer
• removed styled-components that were used in just one place

⚠️ You may notice that some tests became less "unit'y" and lean towards integration tests. Here's rationale:

• You shouldn't mock selectors or store at all
• The app should be tested the way users interact with it
• You shouldn't mock components

⚠️ Also, some tests have a lot of duplicate setup code. I've spent a while trying to make this better, but in the end it comes to a choice whether a test is self-contained&easily readable or there's no duplication. I chose readability. If you see a concrete way of achieving both, I'd be glad to hear!

[KuznetsovRoman]

Looks like react-testing-library does not give a nice way to query by class name as they don't want us to do it:

Note that using this as an escape hatch to query by class or id is not recommended because they are invisible to the user

If you find yourself using container to query for rendered elements then you should reconsider! The other queries are designed to be more resilient to changes that will be made to the component you're testing

It does not seem to worth fixing these in old tests, though.