Refactors all instances of .concat() to use array spread operators instead

Closes #1459

This PR ports across GitHub’s caching approach to the GitLab platform API.

Refactors file system of platforms and adds test to ensure GitHub and GitLab have same module.exports signatures.

Adds paginated results capability for gitlab and enables it for getFileList. This should enable all files when using APIv4.

Hopeful this c-l-o-s-e-s #962 & #968