Handle case when `reviewers` marker is missing

[danuker]

When a PR/issue has no reviewers specified in the description, the hook should just update the status, not the assignees. request review from the configured default.

[adiroiban]

I think that the important functionality here is that when no "reviewers" marker is found the robot should fallback to a repo or organization default list of reviewers.

---

Yes, you can't assign to a team, so you not try.
But what you can do, is that instead of assigning the PR to a person, you should request a review from a person or a team.

---

The current "assignment" behaviour is from the old times when there was no concept of "reviewer" for a PR.

At the beginning of GitHub, I think that even inline comments were missing.
You only had the general comment.