WIP: Add rake gitlab:mr:checkout to checkout a MR

GitLab has multiple components, and these need to be stopped and started in a correct order when switching to another branch, while also some steps might have to be taken too. These steps include running yarn, or bundle.

rake gitlab:mr:checkout aims to provide a failure proof way of going from one merge request on gitlab-org/gitlab to another. Internally it does this by using the merge-requests/<iid>/head refspec with a local mapping. Beside the Git aspect, this command depends on Runit as it provides great control over the services. For the command reliability is proffered over speed, which means that lots of tasks are executed which might or might not have to be executed.

Closes #288