Add command for relabeling issues from Zulip

[LeSeulArtichaut]

As suggested in today's Prioritize WG meeting, being able to relabel issues from Zulip could be helpful, e.g. for prioritizing issues as per the prioritize-WG or labeling during meetings, without leaving the Zulip chat.

The command syntax could probably follow the existing @rustbot modify labels on GitHub, but may need to take an additional link-to-the-issue parameter, e.g. @rustbot modify labels for #XXXXX: .... If absent, it may look at the topic name and get the link from there (e.g. all topics names for I-prioritize issues have the link to the issue inside them).

This issue has been assigned to @LeSeulArtichaut via this comment.

[Dylan-DPC-zz]

cc @rust-lang/wg-prioritization

[spastorino]

To add a bit more about our requirements, the ideal would be that we issue a command on Zulip and have the issue label I-prioritize be removed, the passed priority label added to the issue, a message posted to GitHub that looks like ...

Assigning P- as discussed as part of the Prioritization Working Group process and removing I-prioritize.

and also the topic name changed from for example I-prioritize #72896 ICE when structurally matching a struct to P-critical #72896 ICE when structurally matching a struct if P-critical was the label added of course as it was in this issue :).

[LeSeulArtichaut]

I think @Elinvynia was willing to implement this.

[spastorino]

Maybe this #12 is useful.

[LeSeulArtichaut]

I think there should be a relabel command available on Zulip whatsoever, to be able to change any label, not just labels for prioritization. But I agree that, a triage command equivalent to #12 would also be great.

[Elinvynia]

Hello, I've finally had some time to look into it, I'm not sure what the priority is but if it's high anyone is free to use my notes:

The relabel.rs command parser may be reused for efficiency.
Creating the handler by itself is simple, as it's mostly a combination of notify_zulip.rs and relabel.rs

The hard part is adding some zulip API that would pass the events down the handlers kind of what we have with GitHub (and will have with octocrab SoonTM). Right now the bot only reacts to github events, but can post on zulip.

[LeSeulArtichaut]

@rustbot claim