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)

[juliushaertl]

Actually duplicate of https://github.com/nextcloud/whiteboard/issues/63