Add an option to `NotifyZulipLabelConfig` to automatically resolve the zulip topic when the label is removed

[cyrgani]

For some zulip channels (such as t-compiler/prioritization/alerts), this seems like an useful option to distinguish the unprioritized from the priorized issues without having to visit each topic manually.
On the other hand, a channel like t-compiler/backports may not want the topic to be resolved just by removing the *-nominated label, so this should be off by default.
If this is desired and possible, I will try to implement this in the next days.

@rustbot claim

[Urgau]

If we can get it to not be spam (ie only change the stream name, no message in the stream) and not break links, why not, but @apiraino has probably more opinions than me on this.

[apiraino]

I can't think of a way to implement this without implicitly posting a message ("This topic has been resolved").
Breaking links when "resolving" a topic has been thankfully fixed by Zulip.

In general, I personally don't leverage the "resolve" flag of Zulip (but maybe it's just my workflow). If I want to see which issues still need to be prioritized, I go to GitHub and get the full view. I have to open a browser tab on GitHub anyway so 🙂

Or @cyrgani were you thinking about also other streams for an automated resolve flagging?

[cyrgani]

My thought was that for prioritization (and other channels which could use this), the topic will already have just gotten a message from triagebot ("Issue #xyz's prioritization request has been removed." or similar) and been bumped to the front, so the extra "has marked this topic as resolved." notification message would be less of a problem.

[Urgau]

Note that I'm pretty sure that the notification is not automatic, it has to be done by the one who renames the stream. We could just not send it and just add the check-mark.

The way "resolve" works is by appending a check-mark to title of the stream. Zulip bot then sends a message to inform users, but it's only for convenience.