Extract component base class

This extracts a Component base class, removing a lot of redundancy and improving maintainability (I did this mainly because I found a need to add some code to the instance method, which I would have had to add to every component if I hadn't refactored first).

This also refactors the registry login handling, as mentioned in https://gitlab.com/gitlab-org/gitlab-qa/-/merge_requests/465#note_351431872