Warn user if they're about to save a giant file to experiment or checkpoint

[andreasjansson]

Copying files to temporary directories before upload is still synchronous, and with huge files this can take a lot of time. It's also possible that those huge files are included by mistake.

One potential solution is to warn the user if they're about to save giant files, suggesting that they put them in .replicateignore. This needs some thought, because sometimes giant files are intentionally tracked with Replicate.