Add conflict label to a PR when a conflict happens

[Kobzol]

Bors now reports merge conflicts on PRs. One remaining functionality from homu is to also attach a label to the PR that states that there is a conflict. This should use the existing machinery for removing/adding label(s) to a PR when some event happens.

Corresponding homu code:

• https://github.com/rust-lang/homu/blob/2cc8d2f3e32a28f8de2327a3e892f19576e21200/homu/main.py#L1574
• https://github.com/rust-lang/homu/blob/c5291c586d717f9766e376e65a95156d7cdc1b96/cfg.production.toml#L72