[3.0] Switch JavaScript examples to `fetch`

[shalvah]

Why?

• It's 2018.
• jQuery isn't part of JavaScript. Admittedly, neither is fetch. But at least fetch is included in every modern browser, while jQuery is not.

[shalvah]

Removing from the 3.0 project, as this isn't a real blocker. I need to release v3, and don't have time to work on this myself.