Major refactoring using ESLint with rules from AirBnb and Vue.
Enable most of the ESLint rules and do necessary linting in the code.
Also add more information for rules that are disabled to describe what
they are and why they are disabled.
Allow logging (`console.log`) in test files, and in development mode
(e.g. when working with `npm run serve`), but disable it when
environment is production (as pre-configured by Vue). Also add flag
(`--mode production`) in `lint:eslint` command so production linting is
executed earlier in lifecycle.
Disable rules that requires a separate work. Such as ESLint rules that
are broken in TypeScript: no-useless-constructor (eslint/eslint#14118)
and no-shadow (eslint/eslint#13014).
Integration tests are executed using vue-cli-service with double quotes as following: `vue-cli-service test:unit "tests/integration/**/*.spec.ts"`. Using single quotes (mochajs/mocha#1828) works on macOS and Ubuntu but does not on Windows (tests are not found). Double quotes is the only portable way that works on all three platforms (mochajs/mocha#3136).
