webhook accepted: invalid/incomplete milestones should add comment rather than replace issue body

[miketaylr]

I just realized that we probably don't want to replace the body of an accepted invalid or incomplete issue. Rather, we want to add the invalid/incomplete message as a comment. If we re-train our ML bot, we're going to skew results. And we'll also lose possibly interesting reports for future analysis.

For rejected issues, we absolutely do not want report body content -- these are different though.