Code audit (post-merge) code review workflows
### Problem to solve
At some time after a collection of code changes have merged, I want to be be able review them as a collection to audit the code quality of an entire feature.
### Further details
This builds on the primitives proposed in https://gitlab.com/groups/gitlab-org/-/epics/1192.
This epic tracks the workflow improvements needed to start a code review on commits that have already been merged.
### Proposal
Provide a clear and obvious workflow for:
- starting an audit
- selecting the items (commits, files etc) to be audited
### Links / references
epic