Run minimal visual regression testing in MRs

When creating an MR, the visual job runs in every pipeline and tests all of GitLab UI's components.

Generally, an MR only changes a single component though. There is a possibility that one component is being used in others, and that changing it would affect other visual tests indirectly. But in most cases, it should be reasonable to run visual tests only for the component being changed. At least until the initial approval at which point we should run a full pipeline to make sure all visual tests are still green.

Let's improve our CI setup so that MR pipelines only run storyshots on the changed *.stories.js files by default.