Closes gh-4673

(cherry picked from commit 73415da)

The change in gh-4603 made the object returned by `elem.data()`
a prototype-less object. That's a desired change to support keys
colliding with `Object.prototype` properties but it's also a breaking
change so it has to wait for jQuery 4.0.0.

A 3.x-only test was added to avoid breaking it in the future on this
branch.

Fixes gh-4665
Ref gh-4603
Closes gh-4666

This forbids unnecessary `eslint-disable` comments.

Ref gh-4095
Closes gh-4520

(cherry picked from 46f9810)

Closes gh-4678

(cherry picked from commit 88eb22e)

Closes gh-4684

(cherry picked from commit a62309e)

The "jQuery.ajax() - JSONP - Same Domain" test is firing a request with
a duplicate "callback" parameter, something like (simplified):
```
mock.php?action=jsonp&callback=jQuery_1&callback=jQuery_2
```

There was a difference in how the PHP & Node.js implementations of the jsonp
action in the mock server handled situations like that. The PHP implementation
was using the latest parameter while the Node.js one was turning it into an
array but the code didn't handle this situation. Because of how JavaScript
stringifies arrays, while the PHP implementation injected the following code:
```js
jQuery_2(payload)
```
the Node.js one was injecting the following one:
```js
jQuery_1,jQuery_2(payload)
```
This is a comma expression in JavaScript; it so turned out that in the majority
of cases both callbacks were identical so it was more like:
```js
jQuery_1,jQuery_1(payload)
```
which evaluates to `jQuery_1(payload)` when `jQuery_1` is defined, making the
test go as expected. In many cases, though, especially on Travis, the callbacks
were different, triggering an `Uncaught ReferenceError` error & requiring
frequent manual re-runs of Travis builds.

This commit fixes the logic in the mock Node.js server, adding special handling
for arrays.

Closes gh-4687

(cherry picked from commit 7b0864d)

The "focusin on document & window" test didn't cleanup `focusout` handlers
on `window` & `document`. This has been fixed.

Ref gh-4657

Closes gh-4685
Ref gh-4642
Ref gh-4647

(cherry picked from commit dc06d68)

Closes gh-4686

(cherry picked from commit 1a7332c)

iOS 8-12 parses `<noembed>` tags differently, executing this code. This is no
different to native behavior on that OS, though, so just accept it.

Ref gh-4685
Closes gh-4694

(cherry picked from commit 11066a9)