Prevent highfive from assigning PRs to PR author

[KiChjang]

Right now, if a collaborator opens a new PR, there is a chance that he/she will be randomly assigned to review it. We should check to make sure that the PR author isn't being selected as the reviewer for his/her own PR.

[wafflespeanut]

Right, this should be done. And, it could still be improved to cover other cases:

• Post a warning comment if the PR author approves the PR
• Don't assign PR authors even if comments like r? @PR-author are posted
• Revert assignee when PR authors are assigned explicitly (looks pretty strict, but we don't usually assign the authors to their own PRs. So, it might be worth to have it)

Thoughts?

[Ms2ger]

It is sometimes useful to self-assign explicitly, do let's not have the bot complain.

[wafflespeanut]

Okay, then we don't want the third one for now.

[wafflespeanut]

I'm surprised this is still happening!