WIP: Update pipeline UI to accommodate post-merge pipelines

What does this MR do?

This MR makes a number of small changes to the current pipeline UI in order to support post-merge pipelines (gitlab-org/gitlab-ee#7380). The current pipeline UI looks like this:

This MR makes the following changes:

• In the case of a normal branch pipeline, there are no changes to the existing pipeline UI
• In the case of a "detached" merge request pipeline:
  • The previously added merge request tag is removed
  • In its place, a detached tag is added
  • Instead of showing the branch name, the MR ID is shown with a link
• In the case of a "merged result" merge request pipeline:
  • Same as above, except no detached tag

This information is sourced from the Pipeline list view section in the description of https://gitlab.com/gitlab-org/gitlab-ee/issues/7380#pipeline-list-view, which includes UI mockups showing each of the three scenarios above.

Assumptions

This frontend MR assumes the pipeline API has been updated to include a merge_request property as described here https://gitlab.com/gitlab-org/gitlab-ee/issues/7380#note_143891899:

"merge_request": {   // This hash is present only if the pipeline is for merge request
  "iid": integer,    // Show this iid at the ref name
  "path": string,    // Link to this path from the iid
  "title": string,   // Show tooltip when a cursor hovers the iid
  "source_branch": {
    "name": string,
    "path": string
  },
  "target_branch": {
    "name": string,
    "path": string
  }
}

What are the relevant issue numbers?

gitlab-org/gitlab-ee#7380

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.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #7380 (closed)