Add Triagebot's `concern` capability to the Clippy repository

[samueltardieu]

This is a newly added triagebot capability, which allows registering and then resolving concerns with an issue or a pull request. The concerns are gathered by Triagebot in the issue/PR summary.

Concerns are different from notes: when a concern is resolved, it is striked through in the issue/PR summary, and a link to the comment resolving it is also added, whereas a note can only be removed and then disappears from the summary.

The has-concerns label, which must be created at the time of merging this change, will be automatically set on issues/PRs that have unresolved concerns, and cleared when all concerns are resolved.

changelog: none