Upgrade ESLint to 5.12.0

[jsnajdr]

ESLint upgrade to latest version. Needed mainly to move Babel upgrade in #10810 forward. Old ESLint uses old version of the babel-eslint package that's not compatible with Babel 7.

There is plenty of new ESLint errors reported now, as the rules got upgraded:

• instantiating functions inside connect -- leads to unnecessary rerenders
• JSX a11y warnings
• warnings about legacy React lifecycle methods
• duplicate ES module imports
• legacy string refs in React

I'll leave it up to the Jetpack team to decide whether they want to fix the issues or disable the offending rules.

[jetpackbot]

	Warnings
⚠️	"Testing instructions" are missing for this PR. Please add some
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

Generated by 🚫 dangerJS against 9f25203

[jeherve]

Related:

#10822

And further changes:
#10823
#10825
#10827

[jsnajdr]

OK all these PRs seems to do the same thing. That's a good sign 😄 What are the next steps (cc @brbrr). Can I help with something?

[brbrr]

I'll be happy to take it over :)

[brbrr]

I went through all the ESLint errors and tried to fix all of them. Unfortunately, some of the errors are trickier than other errors, so I disabled some rules with the intention to tackle these errors later with a more incremental approach: #11142

Most rule fixes are grouped by commits for ease of review (you might not want to review the all the changes at once)

[kraftbj]

@brbrr I took a look and everything made sense to me. Would you be game for a rebase and we can get it across the line ASAP after the rebase.

[brbrr]

Rebased! Let's push it through :)

[kraftbj]

Code changes make sense to me. Built fine and the dashboard works as expected with changing settings, etc.