Correctly align resolved discussion text

The header of the `NoteableDiscussion` component is really a media
object[1], but it didn't have the right DOM or styling to correctly
achieve it.

This change wraps all the children of the media object (except the
floated image) in a dedicated `.timeline-content` element, which is
styled to establish its own block formatting context[2]. This ensures
*all* of its children sit left-aligned to the right edge of the float.

In short, the DOM used to look like this:

    .discussion-header
        .timeline-icon
        note-header
        note-edited-text

and now it looks like this:

    .discussion-header
        .timeline-icon
        .timeline-content (with own formatting context)
            note-header
            note-edited-text

This is also now more consistent with the DOM of `NoteableNote`, in that
`.timeline-icon` and `.timeline-content` seem to want to be siblings.

[1]: http://www.stubbornella.org/content/2010/06/25/the-media-object-saves-hundreds-of-lines-of-code/
[2]: https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Block_formatting_context

app/assets/javascripts/notes/components/noteable_discussion.vue

@@ -358,6 +358,7 @@ Please check your network connection and try again.`;
               :img-size="40"
             />
           </div>
+          <div class="timeline-content">
             <note-header
               :author="author"
               :created-at="initialDiscussion.created_at"
@@ -383,6 +384,7 @@ Please check your network connection and try again.`;
               class-name="discussion-headline-light js-discussion-headline"
             />
           </div>
+        </div>
         <div v-if="shouldShowDiscussions" class="discussion-body">
           <component
             :is="wrapperComponent"

app/assets/stylesheets/pages/notes.scss

@@ -550,6 +550,11 @@ $note-form-margin-left: 72px;
   .note-header-info {
     padding-bottom: 0;
   }
+
+  .timeline-content {
+    overflow-x: auto;
+    overflow-y: hidden;
+  }
 }
 
 .unresolved {