🏗️♻️ Refactor data loading

Currently, the way databases are loaded (and, later, will be updated) has some design problems.
Namely, there is no clear separated responsibilities between @cnrs/heimdall, src/router, src/loader.js, src/heimdall.js. Plus, src/events.js is half-assed, and clearly not working as intended (ie. notify views that the model/database is modified).