Add test coverage metrics to jest config and add github action(s) to ensure them

[tracker1]

This is beyond the code strictly speaking, but would be a decent idea to add a test coverage requirement (matching the current coverage level) to the jest configuration.

Then adding a github action to require the coverage is met in pull requests.

Optionally, adding another action, that will update the coverage levels to the current level when merged.

[JamieLivingstone]

Actioned in this PR.

Thank you for the suggestion!