Avoid usage of third party CDN due to legal reasons

## Feature Request

**Is your feature request related to a problem? Please describe.**
Some first party extensions load resources from third party CDNs like jsdelivr. This might violate EU laws like GDPR. As these files can easily be hosted locally this puts Flarum hosters at unnecessary risc. See recent rulings like https://rewis.io/urteile/urteil/lhm-20-01-2022-3-o-1749320/ Note: I am not a lawyer and from a technical perspective I might not agree with such rulings.

This affects the following extensions:
* flarum/emoji: https://cdn.jsdelivr.net/gh/twitter/twemoji
* flarum/core, bbcode, markdown directly by usage of s9e/text-formatter which loads various ressources from jsdelivr like highlightjs (css and js) and s9e (js).

**Describe the solution you'd like**
By default Flarum should provide all necessary resources locally and not rely on third party hosts.

**Justify why this feature belongs in Flarum's core, rather than in a third-party extension**
This would require forking core components just to provide some additional files and a different CDN url. Actually utilizing a third party CDN should be provided by an extension and not the other way round.