Custom expiry time setting for Amazon S3 download links

[iroes]

@RocketChat/core

Cheap VPSs not always have their clock synced (!!!). And it's not easy to fix as it is usually a restricted system wide setting. Users were receiving this xml from AWS (S3) instead of the file they uploaded:

<Error>
   <Code>AccessDenied</Code>
   <Message>Request has expired</Message>
   <Expires>2016-07-23T09:06:09Z</Expires>
   <ServerTime>2016-07-23T09:40:58Z</ServerTime>
   <RequestId>xxx</RequestId>
   <HostId>
   xxx
   </HostId>
</Error>

I suggest changing the 60 seconds expiry time for links to a longer time, for example 10 minutes. A sysadmin cannot ignore a 10-min-delay complain.

[graywolf336]

@iroes Thanks for the pull request! What about changing it to something that is configurable instead of having it hard coded?

[iroes]

Ok @graywolf336, done.