Default issues with any label to "Priority: Eventually"

[jyasskin]

On the theory that if someone with labeling permission (the triage role) adds a label to an issue without also adding a priority, they've probably triaged it in the process.

This won't work for any WG with issue templates that auto-add labels, so I'll need to maintain a list of WGs that have opted into this behavior.

It looks possible to retrieve the collaborators with triage permission using repository.collaborators.edges.permission, but I'm a little worried about the extra rate limit cost of doing that for all repos. With that list, we could say that only labels added by triagers count, which might work for every WG.

cc/ @fantasai