Remove legacy gitlab service

This MR is the last push to remove the old gitlab_service.ts. gitlab_service.ts module depended on deprecated request-promise and it was hard to test.

This MR moves the few methods that were still in gitlab_service.ts into the gitlab_new_service.ts.

The MR is best-reviewed commit-by-commit. Reviewing all changes at once will miss the context.

I can be nudged towards splitting the commits into two or more MRs. I had a slight preference for keeping this as one MR because this change makes to most sense as the full gitlab_service.ts removal.

Removing request-promise from the extension reduces the size of the bundle from 2MB to 726KB 🏎 .

Closes #183 (closed)