Add ability to programmatically set username on comments

Note: I haven't looked at the code to know if this is a legitimate feature request or not; it may already exist.

In #672850: Re-test request comments should come from user who issued them Jimmy mentions that the reason that PIFT makes all of its follow-ups as "System message" or whatever is because Project Issue doesn't (or didn't) have the way to set an author on a follow-up.

If this could be added (or pointed out where it exists), then it would be possible for re-test requests to come as authored by the person who issued them, which both would solve the "My issues" problem and would also let us remove the weird "Re-test request by @user" stuff we do now in PIFT.