refactor: fetch issuables in the new gitlab service

This migrates fetching issuables from the old gitlab_service.ts to the new gitlab_new_service.ts. This was the major remaining part blocking us from fully migrating to the new service which is using fetch rather than deprecated promise-require.

This migration will help us with using virtual workspaces and VS Code in the web #479 (closed).

The MR moved the production code and tests. There are a few new tests.

The logic is covered by both unit tests and integration tests. I also tested the extension manually. I have high confidence that there is no regression in this MR.