Make stale short link expiry optional

[igorwwwwwwwwwwwwwwwwwwww]

What is this feature?

This expands on the work done by @gelicia in https://github.com/grafana/grafana/pull/86003. It allows stale short link expiry to be disabled by setting short_links.expire_time to a negative value.

Why do we need this feature?

A common pattern is to generate a short link and post it to an issue. This short link may not be accessed right away. Yet we will still want to retain it, in case someone comes looking for it later on.

For this reason, we want the option to disable the link expiry behaviour.

Who is this feature for?

Devs and SREs who use short links to link back to the sources of their analysis in issues and incident reviews.

Which issue(s) does this PR fix?:

• https://community.grafana.com/t/short-link-shows-404-after-some-time/74297
• Related: https://github.com/grafana/grafana/issues/83009

Special notes for your reviewer:

I could use some help actually testing this change, as I don't have a viable testing setup right now.

Please check that:

• [ ] It works as expected from a user's perspective.
• [ ] If this is a pre-GA feature, it is behind a feature toggle.
• [ ] The docs are updated, and if this is a notable improvement, it's added to our What's New doc.