GitlabManager => Move instance to singleton

...mmander/assignment/subcommands/AssignmentCreateCommand.ts

NodeApp/src/commander/auth/subcommands/AuthTestCommand.ts

...c/commander/exercise/subcommands/ExerciseCreateCommand.ts

NodeApp/src/helpers/AccessesHelper.ts

NodeApp/src/helpers/GlobalHelper.ts

NodeApp/src/managers/GitlabManager.ts

NodeApp/src/managers/SessionManager.ts

NodeApp/src/shared