Navigate to methods/functions in merge request diff

• When reviewing a merge request diff, navigate to specific methods or functions in the changed files.
• This is an enhancement on top of the new design here: https://gitlab.com/gitlab-org/gitlab-ce/issues/29778.
• GitHub has a similar feature: https://github.com/blog/2407-quickly-review-changed-methods-and-functions-in-your-pull-requests.