jQuery not defined error with Markdown enabled

[andreasnrb]

Steps to reproduce the issue

1. Use plugin Elementor
2. Activate Markdown support in JetPack
3. Edit page with Elementor

What I expected

No error messages in console.

What happened instead

jQuery not defined.

Propsed fix.

Don't assume jQuery is loaded before calling it. See

jetpack/modules/markdown/easy-markdown.php

Line 453 in ce014ac

jQuery( function() {

[jeherve]

I ran some tests, and it appears that Elementor itself removes most scripts that usually load on the edit screen, including jQuery, to then reload it later on its own. To remove all those scripts, they do the following:
https://github.com/pojome/elementor/blob/0cfd51b929e9bbd8ddcd495350d443767a863124/includes/editor.php#L105-L109
elementor/elementor@5879b80

Jetpack's Markdown module, however, adds its JavaScript by hooking into another hook, after_wp_tiny_mce, to indicate that its script must load after TinyMCE scripts, that also rely on jQuery.
Since Elementor does not remove anything that's hooked there yet, it creates the issue. I created a PR here to let the Elementor folks know and offer a fix:
elementor/elementor#5686

Closing this issue now.