Dry-check should check for minimum `write` access to `<repo>` in `bors.<repo>.review = true`

[jieyouxu]

See e.g.#1841

If a team has bors.rust.review = true, they can r+ PRs on rust-lang/rust. Surely if they can r+ PRs they should at the minimum have write access to said repo? Otherwise, the team won't even have triaging perms.

[jieyouxu]

(Note: I'm not sure what coherence checks this repo has)

[Kobzol]

Hmm, I guess the issue is that bors permissions are kind of opaque to team. It treats rust as just an arbitrary string, and there's no connection to the rust-lang/rust repository in team. I guess that we could hardcode it though.

Also I wanted to move the bors permissions from teams/users to the repository itself, to make it clearer who has permissions for a given repository.