Store/sync images in canvas data #107

Currently, we don't store images
users can paste them in pads, but they won't show up for other users accessing the same page or on page reload

Data for a canvas looks like this when empty:

The files dict in vanilla Excalidraw is filled with an encoded version of the file's data

For pad, we should instead a fast and reliable storage, very likely a classic CDN setup that takes any pasted file, uploads it, and gives back the link for it

The CDN must be protected behind auth
We can also investigate file formats other than images like videos, gifs or even PDFs

If anyone is interested to work on this, let me know and we can discuss it!