Refactor isHighlight()

Clarify the logic a little.

h/static/scripts/directive/annotation.js

@@ -216,10 +216,11 @@ function AnnotationController(
       // example there's no model.highlight: true.  Instead a highlight is
       // defined as an annotation that isn't a page note or a reply and that
       // has no text or tags.
-      var targetLength = (model.target || []).length;
-      var referencesLength = (model.references || []).length;
-      var tagsLength = (model.tags || []).length;
-      return (targetLength && !referencesLength && !(model.text || tagsLength));
+      var isPageNote = (model.target || []).length === 0;
+      var isReply = (model.references || []).length !== 0;
+      var hasText = (model.text || '').length !== 0;
+      var hasTags = (model.tags || []).length !== 0;
+      return (!isPageNote && !isReply && !hasText && !hasTags);
     }
   };
 

h/static/scripts/directive/test/annotation-test.js

@@ -683,7 +683,7 @@ describe('annotation.js', function() {
         assert.isFalse(vm.isHighlight());
       });
 
-      it('returns 0 for page notes', function() {
+      it('returns false for page notes', function() {
         annotation.$highlight = undefined;
         // Page notes have no targets.
         annotation.target = [];
@@ -696,7 +696,7 @@ describe('annotation.js', function() {
 
         var vm = createDirective().controller;
 
-        assert.equal(vm.isHighlight(), 0);
+        assert.isFalse(vm.isHighlight());
       });
 
       it('returns false for replies', function() {