Start using ESLint for adblockplusui repository

[kzar]

Background

With #3692 we're adding a base ESLint configuration to the buildtools repository. Once that is finished we want to start using the configuration for the adblockplusui repository.

What to change

• Update the buildtools dependency to include the ESLint configuration.
• Add an ESLint configuration to adblockplusui which inherits from it:

  {
    "extends": "./buildtools/.eslintrc.json",
    "root": true,
    ...
  }
  

• Make the necessary changes so that linting passes.
• Add a note in the README explaining how to lint the code (eslint *.js lib ext).

[kzar]

(Blocking #4864 since the way we're declaring the ext variable in messageResponder.js breaks when we start using let for the variable in the adblockpluschrome code.)

[kzar]

(The changes required for all adblockplusui code to pass are absolutely huge, so I've started off with the messageResponder which is what's blocking #4864.)

[greiner]

See also #4801

[abpbot]

A commit referencing this issue has landed:
​Issue 4871 - Start using ESLint for adblockplusui