Add option to presign public S3 URL

[PseudoResonance]

This adds support for presigning public S3 URLs so that you can use the S3 redirect feature without having to make the entire bucket public. The expiry time is also configurable, between 1 minute (min for Amazon S3 I think?) and 7 days (max S3 value).

[PseudoResonance]

One thing to note, with the current design, this makes the browser/client caching useless. Each time the user tries to download an image, the URL will change and they will download it again. This should probably not be used if you are being charged by bandwidth.
This can be resolved by caching the URLs on the server side and returning the same URL if it's still valid for a while. Perhaps a min expiry config can be added to invalidate and recreate URLs that will expire too soon.

[PseudoResonance]

There we go. I'm not sure if there's any changes you'd like, but I tried to copy the existing media cache code, but modify it for URL caching instead. It feels a lot snappier to scroll up/down in Matrix now, because I don't need to constantly redownload everything, including the profile pictures.