notify-zulip.<LABEL> should also handle IssuesAction::Open and treat it similar to IssuesAction::Labeled

[fmease]

Context: rust-lang/rust#140397 (comment)

Example: Someone adds various labels to a to-be-submitted PR and then opens it. Then, the notify_zulip handler should trigger all registered actions matching the added labels as if they were added after the PR submission. Right now, triagebot simply ignores them and one needs to remove and readd the relevant labels to trigger the actions.

Such a situation happened here: rust-lang/rust#137899. Of course, fixing issue #1184 would also solve this specific case, in a different way.

However, the general issue affects all notify Zulip actions, not just {beta,stable}-nominated (e.g., I-types-nominated comes to mind), so it's worth fixing regardless (and it'd be a lot easier to fix than #1184).

cc @apiraino