Update all open Pull Requests when user install this app.

[sotayamashita]

Additional context

• wip/app@d6603bb

[mooocer]

I agree. When I installed the app, I expected it to also have labeled the existing PRs. This should be prioritized.

[sotayamashita]

@MishuVS I understand its importance but I do not have time now. I would appreciate if you could create a PR for it. 🙇 Is it possible?

[mooocer]

@sotayamashita I don't have any experience with building Probot apps so far and am not sure about when I'll get time to do this, sorry :(
And I meant it should be prioritized but doesn't have to be done immediately. Adding a priority label or something, in case a new contributor wants to work.