CLC-1016 Convert embedded URLs in tasks/notifications to be clickable

https://jira.media.berkeley.edu/jira/browse/CLC-1016

n.b. I noticed just before committing this that Angular actually has this functionality built in! 
http://docs.angularjs.org/api/ngSanitize.filter:linky

So we may not need a custom directive after all. However, I was not able to get 'linky' working with their dirt-simple example. Could be an angular version issue, or could be bad docs. Committing this anyway, but it may be possible to replace all of this with one or two LoC, which would be great.