Improvements to the create merge request alert after push

From gitlab-design#441 (closed). This alert/notice is great because it's contextual and timely.

image

However, there are a couple of small improvements that can be made:

  1. ⚡️ Add notice in all screens, not only in the project.
  2. ⚡️ Allow dismissing the notice.
  3. ⚡️ Rename button from “Create…” to “New merge request” and change it to green