Restore the visual diff functionality

AngularJS has killed the ng-bind-html-unsafe directive, which we used
for displaying the visual diff. (See here: https://github.com/angular/angular.js/commit/dae694739b9581bea5dbc53522ec00d87b26ae55 )

Update the code to work with the new way.

Fixes #807.

h/js/controllers.coffee

@@ -494,8 +494,8 @@ class App
     , 5000
 
 class Annotation
-  this.$inject = ['$element', '$location', '$scope', 'annotator', 'drafts', '$timeout', '$window']
-  constructor: ($element, $location, $scope, annotator, drafts, $timeout, $window) ->
+  this.$inject = ['$element', '$location', '$sce', '$scope', 'annotator', 'drafts', '$timeout', '$window']
+  constructor: ($element, $location, $sce, $scope, annotator, drafts, $timeout, $window) ->
     threading = annotator.threading
     $scope.action = 'create'
     $scope.editing = false
@@ -611,6 +611,15 @@ class Annotation
         if annotation? and drafts.contains annotation
           $scope.editing = true
 
+    $scope.$watch 'model.$modelValue.target', (targets) ->
+      for target in targets
+        if target.diffHTML?
+          target.trustedDiffHTML = $sce.trustAsHtml target.diffHTML
+          target.showDiff = not target.diffCaseOnly
+        else
+          delete target.trustedDiffHTML
+          target.showDiff = false
+
     $scope.$watch 'shared', (newValue) ->
       if newValue? is true
         $timeout -> $element.find('input').focus()