Fix #358 - migrate to S3 policy v4 to support AWS4-HMAC-SHA256

[rubenvandeven]

Fixes #358 so that it works on more AWS regions. I hope it fits your development guidelines, I'm not too acquainted with JS. For some reason I still didn't manage to get it running on non-Amazon S3 services.

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

[welcome]

🎉 Thanks for opening this pull request! Please check out our contributing guidelines if you haven't already.

[catarak]

Thank you so much for working on this! I think it's okay to not get this to work for non-Amazon S3 buckets at the moment—just opening this up to different regions is super useful, as folks are running into this issue often!

[catarak]

I tested and changed this a bunch but it all seems to be working now!

[rubenvandeven]

Still quite some changes, but great you managed to get it working!