[wip] Remove CoffeeScript ref variables

[kevinsawicki]

• Cleans up the ref, ref1, ref2, etc. variables left over from the CoffeeScript migration.
• Migrates to using const and let over long var lines at the top of several functions.
• Migrates to using forEach, find, filter, map over for loops in several places.

Before

BrowserWindow.fromWebContents = function(webContents) {
  var i, len, ref1, window, windows;
  windows = BrowserWindow.getAllWindows();
  for (i = 0, len = windows.length; i < len; i++) {
     window = windows[i];
     if ((ref1 = window.webContents) != null ? ref1.equal(webContents) : void 0) {
       return window;
     }
   }
};

After

BrowserWindow.fromWebContents = function(webContents) {
  return BrowserWindow.getAllWindows().find(function(window) {
    const contents = window.webContents;
    return contents != null && contents.equal(webContents);
  });
};

Refs #4065

[kevinsawicki]

This pull request is somewhat challenging to review due to this type of change so I'm going to review it several times myself and to make sure no unintended behavior was changed.

[kevinsawicki]

I decided to migrate this line given its lack of readability

[YurySolovyov]

Since this PR is against master, can't you use rest args for arguments handling?

[zcbenz]

We can probably use the for of syntax for iterating array, it is clearer and more efficient.

[kevinsawicki]

I'm going to close this out for now and break this up into some smaller pull requests that focus on the following things individually:

• Using rest parameters
• Using destructured assignenment
• Removing temp ref variables