Embed metadata in PRs

[rarkins]

It could be useful to embed "hidden" metadata in PRs using HTML comments. This would work for GitHub and GitLab but not Bitbucket or Azure DevOps.

Examples of use:

• Include a semantic list of updates in each PR. For PRs we consider "ephemeral", this could provide a way of not reopening them. i.e. we check if the closed PR's metadata is the same - i.e. the list of updates is unchanged - and if so them suppress the PR.
• Include backwards-compatibility information in the PR for easier transitioning of app users between major versions

Concerns I have:

• Can we make it so that the BB/AD experience is still OK?
• We should put the metadata at the front of the PR body, not end - otherwise it risks being truncated.
• Should it be machine-readable only to be as small as possible, or something like JSON stringified for human consumption too?

[rarkins]

Another use: embed the job ID when it was created, last updated (branch) and last updated (PR body). Useful for troubleshooting