Add support for organization-wide default configuration

[Urgau]

This PR adds support for organization-wide default configuration, with rust-lang.triagebot.toml as the only one for now.

The default configuration takes the same options and features as normal triagebot.toml, with an optional excluded-repos = [] field to exclude some repos.

No default configuration has being added in the PR.

As discussed in #t-infra > Organization-wide triagebot.toml configuration

[Kobzol]

The implementation looks simple enough (though macros 😿). That being said, I wonder if we should wait for the first use-case? Because replacing at the level of the top-level config might not be enough, i.e. we might want to also change some field of a specific config, and then this approach might have to be probably rewritten from scratch.

Do you have something specific in mind?

View changes since this review

[Urgau]

I have some usecases, but I wanted to avoid mixing them up with the implementation PR.

Specifically I would like to enable org-wide:

• view all comments link
• range-diff
• review changes since
• transfer
• concern?

Because replacing at the level of the top-level config might not be enough, i.e. we might want to also change some field of a specific config

Well, what is PR does, is that repo configs are respected and never overriden by the org-wide config. The repo config always has precedente.

As for specific field overriden, I don't see a usecase for now.

[Kobzol]

Ok, if you only have use-cases for replacing the top-level keys, that's fine by me 👍

[Kobzol]

LGTM with one nit commented above.

View changes since this review