Allow links in issue label descriptions

[nh2]

It would be great if these links would be clickable:

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In nixpkgs, one of the most active repositories on GitHub, we're having a discussion of how to make the stale bot send less email spam (NixOS/rfcs#124 (comment)).

One promising solution we came up with is to make the stale bot not post a message, but only attach a label. This would avoid sending an email as desired. But it has a discoverability problem: Without a bot-posted message, we cannot communicat to our contributors what the stale label means, and what our stale-ness policy is.

This would be significantly improved if we could add a clickable link that explains our approach.

[nh2]

It would be even better if [markdown links](https://...) were allowed, the current 100-character limit for label descriptions could be changed such that it doesn't such that it counts only visible character so that you can use long links.

But this is less important than being able to link at all.