refactor(gitlab_service): convert service to TypeScript

Closes #220 (closed)

This MR is the last of TS migration. After the 600+ LOC gitlab_service is converted, the rest of the codebase can be converted as a part of normal feature work.

This MR cleans up two or three methods in the gitlab_service but mainly tries to leave the code as is and just add the type definitions necessary for the TS compilation to pass.