DRY host-related schemas

[Glutexo]

When #392 gets merged, there will be two schemas duplicating the canonical fact field list.

• In the app.events module, for delete event
• In the app.models module, for HTTP responses

It would be great to DRY this, so any change to the canonical facts list would be reflected in both serializations. Some learning about how Marshmallow handles inheritance and composition is required.