Rollup merge of #150308 - Kobzol:bors-conf, r=marcoieni

Update bors configuration

Updates the configuration of bors to bring it up to speed with homu, in preparation for rust-lang/infra-team#168. Mirrors configuration from homu's [configuration file](https://github.com/rust-lang/homu/blob/master/cfg.production.toml#L46).

This PR also enables reporting of merge conflicts, so that we can test this part of bors on `rust-lang/rust`. The merge conflict reports will be duplicated (until/unless we disable it in homu), but that hopefully shouldn't be such a big deal.

r? ``@marcoieni``

Author: matthiaskrgr