Redact confidential resources in summarize action

Harder than I thought, due to how we implemented it. I need some more time to think about this.

Got some good ideas.

TODOs:

• Make sure Context did redact all the possible confidential fields
• Higher level test (wait for https://gitlab.com/gitlab-org/gitlab-triage/merge_requests/82)
• Update documentation

Closes #98 (closed)