Don't unnecessarily assign additional PR reviewers

[nklaassen]

Description

What happened:
I will often manually add PR reviewers before posting the PR, if they have specific knowledge/context on the work. Whether or not the PR author has manually added reviewers, the review bot will currently add 2 extra randomly-assigned reviewers.

What you expected to happen:
The reviewer bot should only assign the minimum number of randomly selected reviewers. If I have added 1 reviewer, the bot should assign only 1 more. If I have added 2 reviewers and 1 is a code owner, the bot should not assign any more.

Reproduction Steps

As minimally and precisely as possible, describe step-by-step how to reproduce the problem.

1. Create a draft PR
2. Manually assign 2 reviewers.
3. Open the PR (convert out of draft state)
4. See that the reviewer bot will randomly assign 2 more reviewers.

[nklaassen]

Another side effect of this: converting a PR from "Ready for review" to "Draft", and back to "Ready for review" will add 2 more reviewers to the ones already assigned.