Use modern JavaScript syntax throughout adblockplusui

[kzar]

Background

With the messageResponder.js changes #4783 we updated that file to use more modern JavaScript syntax. We're also updating the syntax used throughout the adblockpluschrome repository #4795. For consistency we should do the same with the rest of the adblockplusui code.

What to change

Where possible:

• Replace anonymous functions with arrow functions.
• Replace var variable definitions with let ones.
• Replace for i = 0; i < whatever.length with for ... of whatever.
• Ensure method definitions use ​the new syntax. (E.g. {foo: function(bar) { }} becomes {foo(bar) { }}.
• Replace IIFEs with a simple block. (E.g. (function(global) { ... })(window) becomes { ... }.
• Ensure that "use strict"; is used throughout.

[greiner]

Duplicate of #4871