Shared infrastructure for Keyman sites

Workflow for making updates to shared-sites

1. open a pr on shared-sites, e.g. chore/v0.4
2. update build.sh on a PR on the affected site to refer to the PR branch name, and ensure it passes all CI tests
3. approve, merge the pr on shared-sites
4. make a release on shared-sites, tagging with v0.4
5. patch the site PR's build.sh to refer to v0.4, wait for CI tests to pass.
6. approve, merge the pr on the site.
7. eventually, update other sites to refer to v0.4, as needed (without waiting too long, as it will stagnate, perhaps this could be a sprint task?)