Every other package in this repository is installed
with yarn; yhub-server was the only one on npm,
with its own package-lock.json.
To ensure consistency it now uses yarn like the rest
of the project.
package-lock.json is replaced by yarn.lock, a packageManager
field is added, and the Dockerfile, CI workflow, Makefile,
helm init-db job and the documentation move from
`npm ci` / `npm run init-db` to
`yarn install --frozen-lockfile` / `yarn init-db`.