Updated href with angular expression to ng-href

Per [this doc](https://docs.angularjs.org/api/ng/directive/ngHref), any href that contains an angular expression should be an ng-href.

I did `grep -ril ".href=.\+/{" views` and the only results were this file (options/social/challenges.jade) and shared/tasks/lists.jade, but lists applicable a tags were commented out, so I left them alone.