Update merge request widget to accommodate post-merge pipelines

What does this MR do?

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

Requirements	Mockups
Merge request is merge-able	We will show merge widget as normal (no changes)
Merge request is not merge-able	We will show merge widget as normal (no changes)
Merge request is being merged	Merge button has spinner and states Merging (no changes)
Fork merge request Warning Merge request pipelines are enabled but will not activate due to this being a fork merge request Warning merge button (Merge when pipeline succeeds will also be warning colored as we rely on a normal branch pipeline in those cases)
Invalid merge request as target-branch advanced (linear history) merge button disabled warning target branch has advanced, update source branch and retry merge
Invalid merge request pipeline as target-branch advanced (non-linear history) merge when pipeline succeeds button (which triggers new pipeline when pressed) warning target branch has advanced, retry merge
Merge request pipeline failed merge when pipeline succeeds button (which triggers new pipeline when pressed) warning pipeline failed, retry merge or fix failure
Merge request is converted from WIP to non-WIP	Merge request pipeline is automatically triggered merge when pipeline succeeds button is shown (no changes)

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