Load external api script from provided domain

[symjk]

This change improves the auto-import of the Jitsi API script file, which was previously hardcoded and loaded from https://meet.jit.si/external_api.js. This could cause problems with differing versions or a content security policy when using the library with a custom Jitsi Meet instance via the domain prop.

The script is now loaded from the domain specified in the prop with a fallback to the previous url should the script file not be available