actions: check shortlog

[oliver-sanders]

Add an action which checks that all commits are identified with committers in the contributors log via configured aliases in the .mailmap file whenever the CHANGELOG.md file is edited.

We could potentially run this for every PR, however, it requires checking out the entire repo[1].

Fixes some minor discrepancies.

[1]: There is a potential halfway house of checking out the last N commits then testing that those commuters exist in the contributing file rather than testing exact equality but this is starting to sound like more bother than it's worth.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.py and
  conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• Already covered by existing tests.
• Does not need tests (why?).
• Appropriate change log entry included.
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.

[oliver-sanders]

May have to use ${{ github.sha }} rather than ${{ github.ref }}, not sure.

[oliver-sanders]

github.sha seems to work fine, just tweeking the mailmap to fix the last error.

[oliver-sanders]

Switched to running from cylc/release-actions.

[oliver-sanders]

Added contributor check to the stage 1 workflow.