Allow the expiring time of S3 links to be configurable

[rocketeerbkw]

There are a few places in the api/ui where Lagoon stores and retrieves files. Currently these are environment backups, task files and insights. The files are retrieved directly from S3 using a signed/short-lived link, meaning no Lagoon authorization is performed once a link is generated. The current system generates a link valid for 5 minutes.

If we make improvements to the UI, we should be able to shorten the time links are valid by quite a bit. In order to prepare for that, and to allow Lagoon users to decide for themselves how long it should be, we can move the currently hardcoded expire time to an environment variable.