Serve default loader by server

[niccokunzmann]

The default circular loader was created to load the calendar while it starts.
The tests should not load this from the Internet and local installations should not load that from the Internet. If people embed the calendar, maybe, the loader should be hosted by them.

Add options for the source:

• loading animation served by GitHub
• loading animation served by OWC
• own loader URL

---

We're using Polar.sh so you can upvote and help fund this issue. We receive the funding once the issue is completed & confirmed by you. Thank you in advance for helping prioritize & fund our work.

[niccokunzmann]

If we use these loaders, they are CSS and easy to add: https://css-loaders.com/