Pull request support for reviewing commit-by-commit (BB-14731)

The reviewers toggle to the commits section to review changes commit by commit. Comments made on commits are not associated with the pull request, which causes two problems. First, the comments do not show up in the Activity section of the pull request - requires QA system to have records for the code reviews and would be much more easier to use the pull request Activity section. The second problem, and this is worse, is that comments on commits are lost if the developer rebases the commits in the pull request.This problem could be fixed by providing an overview mode that is something like "git log --reverse -p target_branch..pull_request_branch", ie all the changes in one screen like the current overview but organised commit by commit.