Resolve "Add keybinding for next/previous file in merge request diff"

What does this MR do?

Introduces keyboard navigation to next/previous diff file in MR review as per https://gitlab.com/gitlab-org/gitlab-ce/issues/48798.

In addition to that:

• Persists state.currentDiffFileId for easier navigation within MR
• If a row gets highlighted in a diff file (by clicking or by navigating to a URL, containing reference to a row in a file), state.currentDiffFileId becomes set to the reference the corresponding diff file. The same is true when user navigates to a URL without line number, but only with file hash in the URL.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/48798

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