The Svelte code should be smart enough to build fully qualified project IDs

Problem/Motivation

Project IDs are always fully qualified in the format SOURCE_ID/LOCAL_ID. Because the Svelte code is too stupid to realize this, project IDs are passed to it already fully-qualified by the backend. But that really shouldn't need to be the case; the front-end knows what the source IDs are, so it should be able to construct fully qualified project IDs on its own.