Simplify storage of draft tags

These were being converted from view model to domain model format before
being stored in drafts, and back again after being retrieved, which of
course is pointless.

h/static/scripts/directive/annotation.js

@@ -116,7 +116,7 @@ function restoreFromDrafts(drafts, permissions, domainModel, vm) {
     } else {
       domainModel.permissions = permissions.shared(domainModel.group);
     }
-    vm.form.tags = viewModelTagsFromDomainModelTags(draft.tags);
+    vm.form.tags = draft.tags;
     vm.form.text = draft.text;
   }
 }
@@ -140,7 +140,7 @@ function saveToDrafts(drafts, domainModel, vm) {
     domainModel,
     {
       isPrivate: vm.isPrivate(),
-      tags: domainModelTagsFromViewModelTags(vm.form.tags),
+      tags: vm.form.tags,
       text: vm.form.text,
     });
 }

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

@@ -1277,7 +1277,7 @@ describe('annotation.js', function() {
 
       it('uses the text and tags from the draft if present', function() {
         fakeDrafts.get.returns({
-          tags: ['unsaved-tag'],
+          tags: [{text: 'unsaved-tag'}],
           text: 'unsaved-text'
         });
         var controller = createDirective().controller;

h/static/scripts/drafts.js

@@ -9,7 +9,7 @@
  *    the drafts service, they're only used to identify the correct draft to
  *    return.
  *
- * 2. `isPrivate` (boolean), `tags` (array of strings) and `text` (string)
+ * 2. `isPrivate` (boolean), `tags` (array of objects) and `text` (string)
  *    which are the user's draft changes to the annotation. These are returned
  *    from the drafts service by `drafts.get()`.
  *