Use server-side pagination for GraphQL queries that return lists

[davidebriani]

All queries that return a possibly unbounded list of resources would be better served by API that support pagination.
This way API clients are free to request only a portion of the list at a time, without having to handle huge responses or put pressure on the server.

For instance, an initial attempt to convert the existing GraphQL queries to be paginated was started in #542 which can be used as a reference for the backend work.

Ideally, the work can be split into several steps / PRs, so that both the backend and the frontend are updated at the same time: otherwise the API change would leave the frontend broken.