Migrate digital ocean spaces to amazon S3

[Riscky]

We use spaces mainly for pxl, and s3 for everything else.
Having our object storage in one place is easier to manage, and S3 is cheaper anyway.

[SilasPeters]

These spaces can be archived or something anyway after chroma has been deployed to production, and is proven to be a stable replacement. chroma uses S3. Thus, pxl's spaces will become irrelevant if pxl will become obsolete.

[Riscky]

Note that we have a few more spaces, for the pxl demo website and exec(ut).
We can drop the pxl demo website, but we should probably discuss with the exec(ut) committee what to do with the old conference pictures.
I suspect we can move these to S3, but let's ask first.