Small cleanup in gr-change-list-item
Small cleanup in gr-change-list-item

+ Resolve the TODO about XSS attacks. Since prop$ causes setAttribute
  to be called, any untrusted content must be escaped.
+ Code review values were capped to +1/-1. Account for any value.

Change-Id: Id828e31ab6a7d61468d78e9ee1f336f96d67e237

WANdisco/gerrit,qtproject/qtqa-gerrit,MerritCR/merrit,MerritCR/merrit,joshuawilson/merrit,gerrit-review/gerrit,MerritCR/merrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,gerrit-review/gerrit,joshuawilson/merrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,MerritCR/merrit,qtproject/qtqa-gerrit,WANdisco/gerrit,MerritCR/merrit,joshuawilson/merrit,joshuawilson/merrit,MerritCR/merrit,MerritCR/merrit,gerrit-review/gerrit,MerritCR/merrit,joshuawilson/merrit,WANdisco/gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,joshuawilson/merrit,WANdisco/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,joshuawilson/merrit,joshuawilson/merrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit