Eslint: Audit global variable usage

[jdlrobson]

We have eslint linting for our JS. Let's add a rule for no-undef for consistency.

Define the environment in .eslintrc.json:

  "env": {
    "browser": true,
    "jquery": true
  },

Make sure all other undefined variables are accounted for (see below)

Checklist

• Enable no-undef eslint rule.
• Define the global environment (browser:
• Make sure --config .eslintrc.json is passed in the lint:js scripts section in package.json
• Beware openlibrary/plugins/openlibrary/js/lazy.js - don't make any changes to minified blocks of JavaScript. Instead disable the rule for these blocks
• Where a variable is not defined, fix the error.
• Where a variable is defined outside the file, disable the rule and note where it comes from
  e.g.

// startTime is defined inside openlibrary/plugins/openlibrary/js/ol.js
// eslint-disable-next-line no-undef
'loadtime':(endTime.getTime() - startTime.getTime()),

[ajluong]

I'm going to take a look at this

[dnguneratne]

I've opened a PR for this and would love your feedback.