Refactor related branches controller action and view template

What does this MR do?

This MR makes changes to the Projects::IssuesController#related_branches action, and the related view (app/views/projects/issues/_related_branches.html.haml). This moves logic out of the view and into the controller, simplifying the view, and making it more straightforward to test.

The logic is made more robust, ensuring that branches that cannot be found (odd, but I have seen it happen) or branches that have no pipeline, are handled without error.

This is a pure refactoring, with no changes to performance, API or interface.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers