Merge Request file tree

What does this MR do?

Changes the changed files dropdown into a filterable tree list.

When searching for a file, it changes into a flat list of files, instead of being nested inside of folders.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14249

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides