Comment on any line in file in a commit

Description

• Allow comments to be attached to any line of any file inside a commit.
• This applies to files that have been deleted. (Commenting on any line within a file that is all red.)
• The applies to files that have been created. (Commenting on any line within a file that is all green.)
• In the screenshot below, you can add a comment with respect to lines 61, 62, etc. But you currently cannot add a comment to lines 60, 59, 58, etc.

Context

This is the first baby step to allow us to start extending the line commenting system to do code review of files outside the context of regular flow of new features, bug fixes, etc., but just a light-weight method of browsing files and leaving comments. Also can be used in documentation review. Details here: https://gitlab.com/gitlab-org/gitlab-ce/issues/19055