Expand ESLint

[gnikonorov]

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ESLint was introduced in https://github.com/pytest-dev/pytest-html/pull/367

This first pass included almost all relevant ESLint sections except for the following:

• https://eslint.org/docs/rules/#stylistic-issues
• https://eslint.org/docs/rules/#ecmascript-6

The following three rules were also skipped, since they involved making broad changes across the JS code. It would be better to submit them as separate PRs:

• https://eslint.org/docs/rules/no-use-before-define
• https://eslint.org/docs/rules/#strict-mode
• ~https://eslint.org/docs/rules/camelcase - PR open in https://github.com/pytest-dev/pytest-html/pull/410~ - Done

I'll be adding all of the above, one PR per bullet point when I find the time to.