Show if a pull request can't be auto-merged

If a pull request can't be auto-merged display that where the status
would otherwise be shown.
Add review comments to the comment count

Review comments are comments made on the combined pull request diff.
Standard comments are comments made on the full pull request. Add those
two numbers together to get a better picture as to the number of
comments on a item of work.
Merge pull request #15 from alphagov/auto-merge

Show if a pull request can't be auto-merged