Update merge request widget pipeline block to accommodate post-merge pipelines

What does this MR do?

Updates the merge request widget pipeline block to handle the three cases mentioned in the Merge request widget pipeline block section here: https://gitlab.com/gitlab-org/gitlab-ee/issues/7380#pipeline-list-view. The relevant UI mockups from this issue have been copied below for convenience:

Requirements	Mockups
Normal branch pipeline Unchanged
Detached merge request pipeline Ref now shows merge request ID/Link before the source branch
Merged result merge request pipeline Ref now shows merge request ID/Link before source branch and adds target branch

This MR assumes the pipeline API will be updated as described in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25679.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25679

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer