Try and pull the checklist from the repo.

Falls back to the one specified in the request.
Document the .merge_checklist.md behaviour.
More accurate name for the template fetcher.
Mark _template_from_request_json as internal.

Its inputs are not useful from outside the pull_request method, as it relies on
the structure of the GitHub AJAX response.
Merge pull request #2 from ms-digital-labs/get-the-merge-checklist-from-repo

Get the merge checklist from repo