Feature Request: Issue Org Invitations about prow HOT 1 OPEN

In response to this:

Migrated from the original issue.

What would you like to be added: It would be nice to have trigger include a bold, explicit invitation to join the org in its ok-to-test message after you've made a three PRs (that is, on your fourth), sample message:

We noticed you've done this a few times! Consider joining the Kubernetes org to skip this step, as well as gain LGTM and other bot rights rights. We recommend asking the approvers on your prior PRs to be your sponsors.

(It would also be nice if we had a document to link to that wasn't all about reasons you can't be a member...)

We can do a query like type:pr is:merged org:kubernetes author:$user before making the comment to determine if someone is qualified - this graphql query has a flat cost of 1:

query {
  search(query: "type:pr is:merged org:kubernetes author:katharine", type: ISSUE) {
    issueCount
  }
}

Alternatively, we can just always comment and leave the user to figure out whether they count, but I think this diminishes the effectiveness over a positive assertion.

If there are issues with doing a count over the whole org, then doing it just on the current repository would be adequate.

Why is this needed: Joining the org is pretty intimidating. On top of that, it is wildly unclear when people are eligible to join, and even if they clearly are, it often doesn't occur to them to actually do so. An estimated 1/3 of our regular contributors are not org members. While this doesn't help contributors who do not make PRs who would be eligible for org membership, it would be helpful to those who do. The real bar is much lower than non-members tend to think it is.

We currently mention that "regular contributors should join the org to skip this step", but it's not at all noticeable and "regular contributors" is wildly open to interpretation.

(shoutout to @liztio for the concept, and @Katharine for the original writeup)

/kind feature
/sig contributor-experience
/priority backlog
/help

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