Add support for side-by-side inline comments

Created by: hszcg

• Only display inline comments for changed lines
• Support reply to existing inline comments
• DO NOT display inline comment for matched lines
  • Because we do not have line_code for matched lines in side-by-side mode
  • And I am not sure where should i put this comment, on the left or on the right?
  • We could support this by modifying in https://github.com/gitlabhq/gitlabhq/blob/master/app/helpers/commits_helper.rb#L108
• DO NOT support adding new inline comments
  • First, we also need line_code for matched lines
  • Second, it may involve more code changes. I could do it, but maybe in another pull-request.

Inline Mode

Side-by-side Mode