Expose merge request pipeline parameters for MR widget

What does this MR do?

This MR exposes the necessary parameters for showing merge request pipeline information in MR widget.

The new parameters will be used as the following table in fronend:

Case	Parameter
Show warning for fork merge request	this.mr.source_project_id !== this.mr.target_project_id && this.mr.merge_pipelines_enabled
Show target branch is advanced and the current result of the merge request pipeline is stale	this.mr.target_branch_sha != this.mr.pipeline.target_sha (Note: pipeline.target_sha is only present if the pipeline is merge request pipeline)

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25983
• https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

This MR has CE backport

• CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26826
• EE: Here

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