Default to dangerous MR merge button

What does this MR do?

• Default accept merge request buttons to dangerous red if they haven't succeeded
• While making the states more stable and flash less, MWBS button is a stable blue

Are there points in the code the reviewer needs to double check?

• Tested with MockCiService, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9250

Why was this MR needed?

Screenshots (if relevant)

Blue MWBS button

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Todo

• EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1313

What are the relevant issue numbers?

Closes #28010 (closed)