Include highlights in annotation creation error

Previously the error message about not being able to create
annotations when logged out only mentioned annotations but,
technically you cannot create highlights either. Change the
error message to include highlights as well.

src/sidebar/templates/annotation.html

 <header class="annotation-header" ng-if="!vm.user()">
-  <strong>You must be logged in to create annotations.</strong>
+  <strong>You must be logged in to create annotations and highlights.</strong>
 </header>
 
 <div ng-keydown="vm.onKeydown($event)" ng-if="vm.user()">