Better library support

[juliushaertl]

Excalidraw support element libraries that can be downloaded from https://libraries.excalidraw.com/

Now currently out integration already supports loading them, but the loaded ones will be lost after page reload.

We should figure out if there is:

• [ ] A way to persist libraries per user
• [ ] Option to preseed libraries (e.g. managed by Nextcloud backend)

https://docs.excalidraw.com/docs/@excalidraw/excalidraw/api/props/excalidraw-api#updatelibrary

We should be able to have our own library items seeded, would need to check if we can persist personal library items if they get added