Expand ESLint

[gnikonorov]

ESLint was introduced in #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 Enable eslint camelcase rule #410 - Done

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