Implemented reviews stats report

* Calculate whether mark is in disagreement state (differs from later core's mark)
* Add report that shows stats for last specified number of days
* Don't treat 'Approve' marks as review marks
* Fix table styles to show ordering arrows

Implements blueprint review-disagreements

Change-Id: I9590d69242b533ba1336384c43a9daa3dfc4e926
Merge "Implemented reviews stats report"