Introduce presenters

[marcoow]

Currently we're using the entities and changesets in the web crate directly. That's of course easy but also means we're coupling the app's HTTP interface directly to the db crate which is exactly what we want to avoid. We can easily address that via introducing presenters. We can fix that relatively easily by introducing the concept of presenters such that for input data (e.g. when creating an entity), we have a presenter that wraps (or converts to) a changeset and for output (e.g. JSON responses), we have a presenter that wraps or is generated from an entity.