attempt-backport: unlabel and clean labeling

[Fishrock123]

Should fix #100 (comment) while also giving a way to see when a PR update has changed the backportability.

This was always on the plans, I just hadn't gotten to it before.

cc @phillipj and maybe also @thealphanerd

I don't really trust myself that it'l work without testing though, but I have a plan on how to test the script, so I'm going to do that next and then rebase this ontop.

[phillipj]

Other than my recent negate comments, this LGTM

[Fishrock123]

@phillipj On second look I think my previous checks were wrong. We only want to remove it if we added it.

[phillipj]

@Fishrock123 not sure I follow you.. If we only want the bot to remove labels it added itself previously, that still means my .includes() comments are valid?

If I understand the current code correctly, labels added by the bot will not be removed since it will prevent removeLabelFromPR() from being reached if the label in question was actually added by the bot.

[Fishrock123]

@phillipj You're right. I read it wrong. Updated.